[jannr]

Rusty and others are right. There are differences: a small number in grammar (such as vosotros--only in Spain, vos--limited to specific regions and a few other, more esoteric ones; pronunciation--only in Spain do you hear the "th" sound for z and c before e or i. In Spain centro sound like "thentro" and zebra sound like "thebra." Elsewhere, these two letters sound like "s": "sentro" and "sebra." Tradicionally Spain had a special sound for "ll", but it is dying out pretty quickly, replaced by the "y" sound used everywhere else: me llamo sounds like "me yamo." There are also differences in intonation and vocabulary between Spanish and Latin American dialects.

So, each region is somewhat different from others. Although it is true that Mexican Spanish is different from Central American and South American, in each country there are at also least some differences. All educated Spanish speakers can understand other educated Spanish speakers, even if they have to work at it a little bit. The local differences are more pronounced as you go farther down the socioeconomic scale, as is true with every modern language. If you learn the "vanilla" Latin American variety, no one will mistake you for a native speaker, but you will be understood everywhere.