How does Google Pull the Description Info ?

[iCyborg]

I am just wondering how Google pulls the description info if a page doesn't have a description tag to be shown in its results ?

Also is it good to have a description tag or to not have? Does Google change the description shown in its results for a page? For example for a query "abc", I am at No 2 with different description and for a query "xyz", i am at no. 8 with different description?

Thanks

[edited by: caveman at 4:01 pm (utc) on Oct. 21, 2008]
[edit reason] Edited for clarity [/edit]

[tedster]

There's an entire team at Google that works on what they call the "snippet algorithm". As you can imagine, the choice of description is important for the success of their search results, and they watch clicks on the SERPs for various versions of the snippet algo very closely as they adjust it.

Yes, the snippet displayed most definitely depends on the search terms. Even with a description meta tag, you will sometimes see a different snippet taken from the page's content, or the ODP description - or even a mash-up of description plus other content.

[Receptional Andy]

As tedster says, the snippet is context sensitive, the goal being to display text that gives additional context to the keyword searched for.

There seems to be a rough priority order for what text is displayed:

1. Open directory description
2. Meta description element
3. Automatically selected snippet of text

If a meta description is too short, shorter snippet can appear afterwards.

Interestingly, Google seems to favour human-written snippets to the automatic selections. Likely this is because it's pretty tricky to isolate an occurrence of keywords and then display it within natural "readable" text. The automatic snippets seem to do a pretty good job though, especially considering that a multi-word search often returns pages where it would be impossible to include matches for all the words within the snippet.

So, the text-snippet is always dependent on the search query. The ODP text and a well-formed meta description seem a lot more stable, and can appear for a great variety of keywords and phrases.

It is definitely a good thing to write your own description, since this gives you the chance to write something compelling about your page and encourage greater levels of click-throughs.

But, some care needs to be taken, since in some instances a low quality meta description can cause your page severe problems.

Google are also trialling a few other additions to snippets, including displaying dates before snippets [webmasterworld.com] .