Nice discussion in Economix by Ewe Reinhardt:
The crucial characteristics of a perfectly competitive market are (1) that all of the quality dimensions of the good or service traded in that market are accurately understood by buyer and seller; (2) that potential buyers have full transparency on the price they will have to pay per unit of that good or service; (3) that it is easy and relatively costless for potential sellers to enter and exit this market; (4) and that there are so many potential buyers and sellers that none individually can affect the market price of the thing being traded.
Read the full article but also look here and then here and here for a medicynical view.
Medicynic 