What is the equivalent in FreeBSD 7.2 to yum/yast/apt-get?

As others have said, there is no substitute for the handbook. In short what you're looking for is portupgrade. With it you can run commands like

portupgrade -Rr     # Upgrade all ports and dependant ports

Read the wiki page on portupgrade. And remember to ALWAYS read /usr/ports/UPDATING.

pkg_add -r or the ports tree :)

see:

http://www.freebsd.org/doc/en/books/handbook/packages-using.html

What you are looking for has two pieces. One is updating the base system, and the other is updating the additional packages.

The first can be done with freebsd-update. This command supports both updating with patches to the current release and upgrading to a new release. Look at its man page for details. It's far easier than installing updates from source.

Updating packages also has two pieces -- updating the ports tree, and rebuilding out-of-date ports. The first can be done either with cvs/cvsup/csup or (my preference) portsnap. portsnap works a lot like freebsd-update but is for the ports tree. Again, see its man page. You can use either portupgrade as described in another answer or portmaster (which I prefer due to lower overhead and simplicity, although it probably doesn't have as many features as portupgrade).