Whats the difference: lifting weights fast or slow?

I've recently joined a gym, I'm very slim body type and can never really put on weight. I've been told to do 3 sets of 8 on my chosen equiptment. However, when I look around me, people are doing their sets really fast. I feel comfortable on the other hand doing mine slow and steady. Is there any difference to the affects? (presumably doing them quicker is less effort because you are lifting the weights for less time. But in this case you could up the weight number and do more sets to equal it out?) Hopefully from you're answers I'll know whether it's more beneficial to do them faster but with higher weights, or what. Thanks!