There are several ways one can start an online poker bankroll from scratch and poker freerolls represent one of the most reasonable ways. If you feel that you have absolutely no money to spare on poker, you can still break into the real money online scene through poker freerolls. Freerolls are MTTs (Multi Table Tournaments) which do not require a real money buy-in (registration is basically free) but award real money prizes. Due to the nature of such tourneys, the prize-pools will usually be rather shallow (there are exceptions to that rule though) and even if you do finish in the money, you’re not likely to take down a significant amount, regardless of the number of participants.
Some poker freerolls are offered to actual real money players as a reward for something (signing up, reaching a set number of FPs etc), while others are completely free to enter, as players are not even required to have made a real money deposit beforehand. Most freerolls are only available to depositing players though.
As far as strategy is concerned, poker freerolls are one of a kind. Because they have no money riding on the affair, most players will adopt a radically different strategy approach to freerolls than to real money MTTs. In a word: they’ll take things much more lightly. Some of them go for the do or die move as soon as the first cards are dealt. They figure that this way they either double up early, which pretty much doubles their chances for making the money, or they’ll bust out, in which case they’ll probably save the time they might’ve spent playing and not making the money anyway.
Past a certain point though, poker freerolls play the same way real money MTTs do. Look for MTT strategy advice to whip your freerolling skills into shape.