Well in Internet Marketing terms it's a reminder to search engines that your blog has been updated. You just send a little message saying that you have new content. Helps get indexed quicker.

I use below but there are many other options and plugins.

www.pingomatic.com

sumbu,

what dotslash said is right, it means telling google to send indexing crawler known also as spider to index your site. It works great for blogs.

beside pingomatic, I also use

Blog and ping | Pingoat

it update my blog within hours after ping. you can use site:yourdomainname to check before and few hours after ping.

cheers

As pointed out it is a way to get the search engines to your blog. What I lot of people forget is that you have a lot of other pages that should be pinged such as your article directory RSS page or your hubpage or squidoo lense.

On my King Ping site there is a service that will spider all the urls you put in to these types of feeds and ping when new content is found.

Rodney

Pinging is a way to let Google or any search engine now that you have updated the content on your site. It is not only for blogs either, when I completely redesign a website or have built a website I always ping it. For one it helps Google update all my on-page SEO and it also helps a new website get noticed faster.

Wordpress use an inbuilt pinging service and can notify severs automatically once you update your blog. But blogger blogs, you have to do it manually, any time you make an update on your blog, you can use pinging services and enter your site's name and ping:

Ping-o-Matic!
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Thanks everyone thats great. I use wordpress for my blog but I can certainly use this for my other sites. Cheers.