GALs- what are they?

[BigBrother at MAIL.UTEXAS.EDU]

I was just looking over John's 68hc000 board and I noticed
there are 3 GALs on the board.  Type: 16V8.

What exactly do PALs and GALs do?  They are some
kind of programmable logic device aren't they?  I checked my
Jameco catalog and saw they are only $3, but the programmer
is $700.00.  Wow!  That is pretty expensive.  Do alternative
programmers exist for less money?