What gets cached?

I was reading a comment in another posting and it started me thinking
about this. Let's say I startup an Oracle server. All my queries are a
little bit (sometimes a lot bit) slow until it gets its "normal" things in
memory, then it's up to speed. The "normal" things would include some
small lookup tables and the indexes for the most frequently used tables.

Let's say I do the same thing in Postgres. I'm likely to have my very
fastest performance for the first few queries until memory gets filled up.
The only time Postgres seems to take advantage of cached data is when I
repeat the same (or substantially the same) query. I don't know of any
way to view what is actually cached at any point in time, but it seems
like "most recently used" rather than "most frequently used".

Does this seem true?
s