A rule utilitarian thinks, before acting, about the consequences of people following that rule. If the outcome is regarded as positive, she might decide that it's good to follow that rule in general, and will apply it in future.
An act utilitarian doesn't generalise the act, but regards it as a single action with a single outcome. She will have to weigh the possible consequences each and every time she acts.
Therefore, rule utilitarianism is considered to be more practicable, countering the anti-utilitarian argument that weighing each and every possible outcome each and every time is just not the way we want to (or can) spend our time.
On the other hand, act utilitarians consider rule utilitarians somewhat dull-witted, for a smart person might think of herself to be able to decide what to do without just applying rules time and time again. Also, blindly applying rules to specific situations can have unforeseen negative consequences that might have been averted by somebody who paid more attention instead of executing a programme.
At the same time, act utilitarians are criticised for their double standards, for they think it is useful if everybody follows "good" rules while they take for themselves the right to decide whether or not it is clever to stick to those rules in a specific situation.
An example: A rule utilitarian drives at night and sees a red intersection light. Thinking "it would have good consequences if people would stick to the rule and not cross red lights, so everyone is safe while waiting for a short while", she would apply that rule to herself and wait for it to turn green. Meanwhile, the act utilitarian might think "well, I certainly hope that people, who aren't me, in general follow that rule and stay put, but as there's no one around who might get influenced by my act, since there's no police around to fine me, and since I would see an approaching car as it's dark, I might as well cross right now."
Sources: There is a paper by Smart which you can find here; I'm pretty sure that's what we read in the seminar where I learned what I wrote. Smart's the act utilitarian.