In laymans terms:
Google crawls your site and then indexes what it sees as a cached version of the page.
The page may change in design before they crawl and then reindex the new page hence the term cache is used as almost a caveat to say that the page may've changed since they crawled it.
If your webpages aren't crawled then they can't be indexed. Making sure your site can be crawled by bots is a priority. Set up a Google Webmaster Tools account and then submit a XML sitemap to help Google crawl and index it.
Hope that helps.

Crawling- Google sends its spiders to your website..
Indexing- Google visited your website and has added you to its database..

Caching- Google toook a snapshot of your website when it last visited and stored the data in case your website went down or if there are some any other issues.

Crawling is the process of an engine requesting — and successfully downloading — a unique URL.
Indexing is the result of successful crawling. I consider a URL to be indexed (by Google) when an info: or cache: query produces a result, signifying the URL’s presence in the Google index.
It means, if we uploaded a new website, then first of all search engine crawler will read the site and after that, it will store all its contents in its Index Data Base in a different format, it will not place content as it was published. As a result, the site will appear in search results for optimized keywords.

Crawling is where search engines spiders / bots move from web page to web page by following the links on the pages. The pages "found" are then ranked using an algorithm and indexed into the search engine database.

Indexing is where search engine has crawled the web and ranks the URLs found using various criteria and places them in the database, or index.

Crawling means google has crawled your site atleast once and your site goes into their crawled database. Indexing means google has crawled your site and found it valuable enough to put it in their indexed pages database.

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"Crawling is where search engines spiders / bots move from web page to web page by following the links on the pages. The pages "found" are then ranked using an algorithm and indexed into the search engine database.Caching is where copies of web pages stored locally on an Internet user's hard drive or within a search engine's database. "

Good tutorial about crawling, indexing and catching, it removed my doubts too, learned too.

Thanks

how to find when our site crawled and index whatever caching we can find easily with time as well as date.

You can check what is Indexed (so also crawled) by going to Google and typing: "site:http://YourSite.com"
All without the " "

You can check what's Cached by going to Google and typing:
"cache:http://YourSite.com"
All with the " "

Indexing- Added your website in to its database after crawling.
Crawling- when the website visited by the spider for checking the websites.
Caching- when the last visited and taking snapshot of the website.

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thanks for well explanation. understood clearly

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* Crawling is where search engines spiders or bots visit from web page to web page by the links on the pages.
* Indexing is where search engine has crawled the web and ranks the URLs found using various criteria and places them in the database.
* Caching is where copies of web pages stored locally on an Internet user's hard drive or within a search engine's database.

Crawling is when google spider visits your sites pages one by one. Indexing is when google will save your sites information into its database.
And in caching it saves a new web page in the google cache database which is read to be indexed.