First off, know how a nofollow and a dofollow backlink helps you differently. Basically for simplicity a dofollow backlink pulls more "weight" than a nofollow. Nofollows are NOT ignored when counting your backlinks, but they are not "worth" as much by far. 100, 200, 500, etc dofollow's is better than 1000 nofollow's. Plus the search engine actually follows the link to your site and checks you out. Dofollows are a good thing, aim for dofollows and settle for nofollows if you must.
Second, understand Pagerank (PR) and how it affects your backlinks. A backlink on a high PR site is worth it's weight in gold. Search Google images for the words "pagerank chart" for a list of charts that show you exactly how much "weight" one link has to another in relation to that pages PR. (sorry, I would link you a great one but I can't due to post limit.)
The higher the PR is of the page linking to you, the high your PR will be. Seek out high PR backlinks, but don't waste them on the low end of a backlink funnel either (more on that below). Save the HIGH PR (5 and up) backlinks for your main site, or at least to a site linking to your site. Some only aim for PR7 and up for their main site, these can be harder to find. It's up to you.
Third, you need to understand how to funnel backlinks. Never ever send crappy low PR spam site backlinks to a site you care about. Ever. This is where all the nightmare stories come from. People buy cheap backlinks or go around spamming tons of links to their main site and lose everything in a Google slap or even an outright Google ban.
Backlink pyramids are wonderful. Backlink wheels are dead and should not be used. Let me explain. Backlink wheels mean you would build 10-12 sites, link them all together, and then link each one to your main site. If you're still doing this, STOP NOW. This is another reason so many people get banned.
Pyramids are natural and occur over time if left alone. This is what Google loves. Pyramiding means you have 20-30 (some people will do 10x this number) very low end sites that link to say 10 decent sites, that then link to say 5 really good sites, that then link to your main site. It builds a pyramid with a solid foundation. Real backlink pyramids should have more levels, with at least 3 and 6-8 being common. Build as many levels as you like but realize your bottom end will be growing and will become harder and harder to funnel backlinks into. If you followed one branch out of a naturally occurring link tree this is what you would naturally end up with. Google loves it.
So how do your backlinks play into all this? Well you send those crappy backlinks we were talking about earlier to the very low end of your pyramid. You can get these easy with little work or money (if you're buying) so most people send hundreds of thousands to the bottom levels. How you get them is up to you. Most of the link packages you see on Fiverr are of this quality - crap. You then backlink your better links to higher tiers of the pyramid and so on and so forth until you end up only sending the very best high quality backlinks straight to your main site.
The usual FAQ:
Q. What about .edu and .gov?
A. These are great, as Google trusts them automatically and gives them a lot of "weight". However fill your backlinks up with these links and you will still be nerfed. Google likes variety. A little .com, a little .edu, a little of everything. Don't go overboard.
Q. How about web 2.0 sites?
A. Google also gives these a lot of trust. There was a time (the golden age) where web 2.0 were like magical gifts send from up above and many IM'ers made fortunes with them. Google realized what was going on however and the rules have changed. Web 2.0 sites still have a place, you can use them for the bottom of your pyramid for example. Google is never going to nerf the actual web 2.0 sites, but your subdomains can be in danger of a nerf. Currently subdomains are still fairly safe unless you just go nuts with spamy links.
Q. Should I still be using forum backlinks?
A. Forum backlinks are the bottom of the barrel. Not only do they ruin good forums but they can really hurt your PR (to say the least) if you have a lot of them pointing to your site. That being said, there are tens of thousands of forums set up now just for backlinking. It's NOT spam, they welcome it. It is still dirty links however. Like web 2.0, they have their place and it's at the bottom. Forum links are usually the cheapest links to buy.
Q. How fast should I get my backlinks?
A. Well, lets be honest. You SHOULD wait and let your backlinks happen naturally. Just search for "backlinks" on the forum and you can find a million ways to get backlinks that all take a good deal of time. And that is how natural backlinks look and work, small amounts over long periods of time. There are (uninformed) people that like to buy their backlinks in bulk of course, I think we all have at some time and most have learned their lesson. It's a practice that's not going away any time soon. If you buy a metric bloody ton of backlinks at once they will all get dumped on your site in a matter of days. This is terrible practice and will get you in a lot of trouble. There are services that let you slowly add backlinks over time, these are the best way to go IMHO but also costly.
Q. Can't I just make all my own backlinks?
A. Sure, I recommend you do this. At least know where your backlinks are coming from and where they are going in your pyramid. You can make your own backlinks in a number of ways, from emailing sites in your niche to hand posting comments on blogs. It takes A LOT of time and you may be dead before you see any real results, but most true whitehat purists will tell you this is the only way to go. I'm not going to argue.
Q. What about buying backlinks on Fiverr?
A. KNOW what YOU are doing here!! They will fill your order however terrible you tell them to. If you're going this route then you MUST understand the difference between crap links and good links AND understand what links need to go where. You can get all sorts of backlinks from fiverr, from ultra high PR to bottom of the barrel forum scrapes. READ the reviews of each seller, try to find customers who waited a few days before posting reviews.
Also note that most Fiverr backlinks will disappear in a matter of days, even high PR ones. This is because most of these users blast your link to social sites and web 2.0 type blogs. Your backlink is only as good as the time it spends on that sites homepage in almost every case. After it gets pushed off by more and more spam your backlink will disappear off the radar. This is even worse if you paid more for High PR links only to have your backlinks fall to a PR 0 page in a matter of days.
Another caveat of Fiverr backlinks is that you cannot control what happens on those sites later. If some Fiverr user is already spamming links to them then I guarantee you those sites will one day be worthless and so will your backlinks. Most sites are filled with useless links then sold. Your links could easily be removed or even worse the site could be flooded with porn, drugs or other no-no's and Google will come after you.
Q. Do LINK (not article) directories still work?
A. Yes and no. If you use a service that submits your link to a billion and one directories then no. Chances are 99% will be nofollow PR0 sites, the page your link is on will almost always be a PR0 no matter what, again these go to the lower-mid end of your pyramid. Yes, in that you can find good high quality directories and PAY for a DOFOLLOW link. These work well. DMOZ is a nice example but there are tons and tons of lesser (and cheaper) directories still worth looking into.
Q. What about ARTICLE directories?
A. This is an art in itself. In general they are the same as pure link directories but you can do a whole lot more with your keyword seeding because you are writing the content. Make sure your content is great, useful, and has a point. The quality of your article really counts. If you're just being lazy and rehashing spun PLR for your articles then you're going to have a really bad time ever getting accepted into decent (good PR) directories. Google reads those articles and decides if they are useful before it counts your link as spam or not. Spam articles = spam backlinks.
Q. How important are my keywords?
A. VERY!!! But here again use variety. If every link coming into your site uses the exact same keyword, then something is amiss. Vary your keywords. However this can be a double edged sword if you're trying to score for certain keywords with your backlinks, then you have to be somewhat focused. Try picking 5 or so and getting some good backlinks built up, then doing 5 or so more. Overall 20-30, even 40-50 different keywords would be a good idea. This takes time to do right.
So in summary; take your time, do it right, know what goes where, and never ever point crappy links to a site your care about.
Now go turn my 2 cents into a fortune.