Macs in the recent era have used two kinds of processors -- ones that are part of the PowerPC family (G3, G4, G5) and ones that are made by Intel (Core Solo, Core Duo, Core 2 Duo, Core 2 Extreme, Xeon, etc). All the current Macs use Intel.
The Intel macs have the ability to run most but not all software written for the PowerPCs, but it is generally somewhat slower and uses more memory. PowerPCs basically cannot run any Intel software.
So three kinds of binaries exist:
PowerPC only -- runs on both, but slower / ram hogging on computers with Intel like the Macbook Pro.
Intel only -- only runs on Intel processors.
Universal -- runs on both at full speed, but is larger than either a PPC only or Intel only binary because both are included.
For the most part, few things outside of games exist in Intel only format yet. Basically, you want Universal or Intel only as much as possible, and/or you need to budget more memory for PPC apps and also make sure they can work on Intel macs (some that use certain features cannot, although they're a small minority).
Hope that helps.
