Yes you can win, but it can be addictive, in my view the people who win the most are the people who play for the longest periods and/or bid higher sums of money.
For example some online casino's I play on for fun. it will cost 25p per spin on a online slot machine for example, but you can select what ever amount you want to bet per spin from 25p - �75 (yes �75 per spin-press of a button). So when people say/or it says online that 'Bob from Bristol won �1,200 is was because he bid's big, loses big and that win in question means he is actually down financially due to huge loses. But they don't tell you that, and 'Bob' wouldn't admit it either. You can win big if you play low bets it just reduces your chances of winning big amounts, for example you will win �1 or �2 not �500.
If you fancy trying online gambling I would recommend the national lottery site, as this is very well regulated, and you know exactly what you are spending, no hidden charges. All games have the same cost so you have the same chance of winning that other people.