top says much memory for File, less for Free

[Water NB <netbsd78%126.com@localhost>]

I have 2G memory, after run few days, top says:
Memory: 1353M Act, 68k Inact, 2000K Wired, 25M Exec, 1243M File, 45M
free

I want to know,
1) How 2G is used?
2G = who? + who? + who?
2) Why so much used for File, so less for Free?
how can I reduce File used?

My system is AMD64 dual core, NetBSD 4.0_STABLE.
Thanks.