Although the term ought to refer to any Greek philosopher from c. 600 bc to c.400 bc, the last year of Socrates' life, it is customarily reserved mainly for those thinkers who attempted systematic cosmologies and were centrally concerned with the nature of physical reality. It thus excludes the Sophists. The preeminent schools with just those concerns were the Milesian or Ionian school, the Eleatics, the Pythagoreans and the post-Eleatic atomists, as well as Empedocles and Heraclitus. Presocratic concerns were as much scientific as purely philosophical: the nature of physical substances, the number of ultimate kinds of thing, the existence of the void, the nature of temporal change (see Zeno's paradoxes), and of course the nature of the cosmos. They included the first atomists (Leucippus and Democritus) and probably the first thinker (Anaxagoras) to explain correctly the cause of lunar and solar eclipses.
http://www.ellopos.net/elpenor/greek-texts/ancient-greece/history-of-philosophy/default.asp Introductory essays on the major Presocratics
http://www.phillwebb.net/History/Ancient/PreSocratics.htm A bibliography on the Presocratics, with links for individual thinkers