I'm very interested in search engines.
Today in a talk I heard that google performs a text search, while more complex engines could rely on the use of metadata, which is apparently not so used by google.
Which is the difference between text search and metadata search?
Could you provide some links where I can go deeper on this subject?
Metadata is 100% text.
The reason why Google doesn't use it is because people tend to lie about their content (not automatically on purpose.)
Now, what Google doesn't use is the Keywords meta data tag (although they may be checking it out to see whether you're a liar...) They do use the other meta tags.
I just wrote a long list of meta tags supported by many systems. I still need to add many more, but out of those that are there the og:image and description and some others are very useful.