Avrohom Yitzchok's answer offers a good web site for some info on common kosher symbols used in U.S.
I would strongly recommend looking at or purchasing The Kosher Supervision Guide from http://www.kashrusmagazine.com/. This lists almost every Kashrut symbol in the U.S., Canada, and I believe they also include many international symbols. Every one has a different standard of kashrut, so, even if you see a kosher symbol on a package, it may not actually be reliably kosher for you. For example, few people I know will trust "Triangle K" because of a few "unusual" rulings by the head of this organization.
Additionally, you need to be updated and informed about "false" kashrut symbols. OU, Star-K and Kof-K, among the top U.S. kosher symbols frequently report products that show an unauthorized symbol, or a change in ingredients, or something else that would make that product non-kosher, despite their having a symbol. The Kashrut site as well as all the individual sites I mentioned would list these problems on their web site and / or in their own monthly magazine.
Despite the seeming complexities that I mentioned, B"H, in the U.S., today, keeping kosher is still far easier than it was 40 - 50 years ago, and earlier. And you can largely thank the internet for making knowledge and access to good information for this.