There are two main CPU makers for PCs, Intel and AMD.
Intel make the Pentium.
For years AMD have been making excellent CPUs (better than Intel in some cases) and they have been cheaper.
AMD have been selling CPUs to small companies, and people who build their own PCs, but found it hard to get into the corporate market (IBM, HP etc), Intel had that all sown up.
In many cases Intel had an exclusive deal with come companies, stopping them using other peoples CPUs.
That is now changing and more companies are starting to use AMD.
You will not notice any difference in running a PC with an AMD from an Intel.
I have a PC that I built a few months ago that is happily running Windows XP on an AMD CPU.