[CentOS] packages

sorry for the newb questions, but I'm new to linux administration, and even
newer to centOS.  I do have some experience with Gentoo Linux.

How can I get a list of packages that are installed in my system?
Is there an equivalent of merge -uDvt world? (This updates the installed
packages with latest releases)
Is there an equivalent of portage in centOS? (The portage site lists all
available packages for Gentoo by 'category').

Thanks for the help.

JD