The Stanford Encyclopedia of Philosophy is a helpful resource for Western and some Eastern philosophy. If you’re just interested in a dictionary-length explanation, each entry begins with a brief overview of the featured term; for those who desire more in-depth explanation, the entries continue in detail. Each entry is refereed by the SEP‘s editorial board before publication.
The Massachusetts College of Liberal Arts offers another good guide: How to Study Philosophy: Tips, Hints, and Resources.
If you’re looking for the “big picture” — and fast! — you may be interested in Spark Notes’ Philosophy Study Guides. Here you will find articles on major Western philosophers and a good number of major works.