There has been some argument over time regarding what was meant by the Ancients when they referred to "The Sea People(s)." Some have said pre-Greek Greeks while others have suggested they were that and more, a loose confederation of mostly-pirates who were also traders and mercenaries-for-hire. Some accounts have them banding together for full-scale invasion. One of the more well known stories of them is outlined on ancient-origins.net and has to do with "The Battle of the Delta: Ramses III saves Egypt from the People of the Sea." Find out more here.
Also, on wikipedia.org, now that the appetite is whetted, read a bit more on them and the controversy that surrounds the Sea Peoples here. And salimbeti.com has some primary sources that mention the Sea Peoples here.
Finally, on ancient.eu, Joshua J. Mark gives a pretty good account of the facts in summary here. Personally, I fall into the camp of the Sea Peoples being refugees from the collapse of both the Myceanean and Hittite empires of the Greek peninsula, Crete, and Asia Minor. Without more definition from the historical record, I don't feel it can be more specific than that.