The two largest Presbyterian denominations in the US are the Presbyterian Church USA (PCUSA) and the Presbyterian Church of America (PCA). However, there are other smaller Presbyterian denominations including the Orthodox Presbyterian Church (OPC) and the Evangelical Presbyterian Church of America (EPCA).
I have listed some differences below and have attempted to be fair (especially since TragicPizza will be reading). Both the PCUSA and PCA share the same heritage and hold to the reformed faith. And you will find many good local churches in both denominations. Personally, I have attended both, and am currently a member of a PCA church.
The PCUSA is the older and larger denomination, and is normally more liberal than the PCA. Princeton Seminary is one of their major seminaries, though I personally know two PCA pastors who went to Princeton.
The PCA is the smaller and younger of the two. It broke away from the PCUSA in the 1980's, and is more conservative. The denomination's seminary is Covenant Seminary in St. Louis, but also supports/works with other seminaries such as Reformed Seminary and Westminster Seminary.
I hope this information is helpful. For more information, you may want to visit the various web sites.