It is a package manager for mac os x (with friends) if that concept is familiar.

Basically, it automates the standard way of building software on unix.

usually, on unix you do

./configure 

make

sudo make install

where the ./configure part may have _lots_ of options

The options may be different on different flavers of unix

Macports

1- ports the software to mac os x (ie the people behind MacPorts)

2 - provides reasonble configure alternatives

3 - makes is possibly to have several versions of a particular program

and easy to switch between

4 - downloads and compiles the software for you

5 - it also downloads and compiles OTHER software, that the one

you really want is dependant on

You need macports + develpment tools

/Björn