The long history of Western Philosophy is usually considered to begin with Thales of Miletus, who was active around 585 B.C., and will probably continue as long as humans exist.
For convenience, it can be divided into three main eras:
Within these eras, the following major historical periods are often identified:
The dates are just rough guides, and the classifications rather arbitrary (for example, the Modern period is sometimes considered to start with the Age of Reason philosophers, and sometimes with the Renaissance philosophers). There is obviously also a certain amount of overlap between these periods.
Also see the Philosophy Timeline for an overview of Western Philosophy, created as one long image to give an idea of the relative scale and the clusters of activity in philosophical thought.