a friend is a comp newbie and is looking to purchse this dell pc but would like some of the terminology explained in simple laymen terminology.

basically she want's to translate....

2.5GHz is the speed of the processor - not the fastest available (they are over 3.00GHz now) but fast enough (I'm still using a 1.4GHz)

256Mb DDR RAM is the memory - the workspace used whilst programs are open.

120Gb Hard Drive is the filing cabinet where programs and documents etc are stored when not in use. More than enough space for all except power users (and those that download music 24/7)
I've got a 40Gb hard drive and that is less than 50% used.

The bigger the number the better.