I'm new to voting for presidents. I only got to vote for 1 president so far.
I want to know what a primary is? Why only certain states get to vote on a guy to run for their party. Also What is a caucus?
Why are these states aloud to vote for someone to be a potential president?

It's not that only certain states get to vote, it is that different states have chosen different ways to vote, and that the votes take place on different days. States chose delegates to support a particular candidate and then the delegates vote for that candidate in the party's nationmal convention. The number of delegates a state gets is roughly proportional to its population. By the end of the primary/caucus season, the election is essentially decided, so those states don't have much sway, although they technically have just as much sway in terms of delegates as a state of the same size that votes earlier.

Primaries are much like a regular election. That is, the election booths are open all day and each vote is counted equally at the end of the day. In caucuses, people gather together at a certain time of day in a given place for each area of the state. They then discuss the candidates and literally stand in their candidates corner. If at least a certian percentage (15% in Iowa, I'm not sure what in other states) of the people in any place are not for a certain candidate, they can either have their vote not count or switch to another candidate's corner. Each part of the state has a given number of points that they can distribute among the most popular candidates in the room. Then all of the results from the regions are collected to see who won how much of the state's delegates. In this system, every vote is not equal and the final percentages are not necessarily the same as the percent of the state that would vote for the particular candidate.

hey thanks methos. I get it now.

One thing I forgot. There are also delegates called unpledged delegates (the pledged delegates are the ones I discussed above). There are approximately 800 of them (as opposed to approximately 3,500 pledged delegates. The unpledged delegates (also known as superdelegates) can vote for whomever they want. They are not bound by any type of primary or caucus.