well, when you buy your cpu and gpu (graphics processing unit or graphix card), there are heatsinks (big pieces of metal) that conduct the heat away from your chip. the paste makes sure there is a good seal between the chip and the heatsink (air is a horrible conductor of heat). the heat sink will already be attached to your gpu, but the one for your cpu will be separate from the chip and already have thermal paste on the bottom of it (it refering to the heatsink).
if you want lower temps, or if you plan on overclocking, it is advisable to remove the thermal paste from the bottom of your cpu heat sink w/ rubbing alcohol and a paper towel and put a dab of artic silver 5 and spread it around w/ a straight piece of thin plastic where the old paste was after the alcohol drys. you dont actually need to spread it around, but i like to
the gpu is alil tricker but not by much, u just gotta unscrew the heatsink/fan from the board and remove the stock paste from the chip and heat sink. again, put a dab of paste on the heat sink and spread it around or just leave the dab in the middle of where the old paste was. then carefully reinstall the heatsink/fan by trying to put it back on w/ a minimal amount of shifting to keep all the paste over the chip.
by dab i mean bb size amount of paste.
u'll can expect about 2-5*C lower temperatures just by doing this simple procedure as your installing your heat sink. if you order it along with the rest of your computer, there probably wont be any shipping.
one of the little tubes should last for like 12-15 applications (specs say 15-25 applications). i've done 6 applications and still have at least half a tube left