Just wondering, what is the common way to find 'similar' forum threads (for example).

Just like on this vbulleting forum and on many other forums many threads contain links to 'similar threads'.

How are the 'similar' items found - based on only the title of the thread or using the fulltext index of the body of posts?

Can someone explain this to me, no php code is necessary, I can write my own as long as I know the steps involved? I mean , do you first break up the title into individual words, then find threads with titles that contain these words? Do you use any type of 'stop words' to filter our common words like 'they, this, the', etc...