I joined 5 years ago when dd was in the school nursery. Here you can get as involved as little or as much as you like. For instance when I had ds, I took more of a back seat. TBH a lot of PTA's are pleased for help, even if only occasionally. We all have different other commitments, whether working, younger children to juggle, non-available partners to help with the children etc.
In our school, each class has one or more reps that attend meetings and liase with class teacher/other parents.
Then there is a chair, secretary and treasurer.
During the course of the year, we organise cake sales (one per year group), Christmas and summer fairs (our big things and the real money spinners). Then we try and have one other event each term, eg a disco, multicultural evening, quiz night, that sort of thing. Not so much for the money, just a social event.
I am more involved than ever now, having taken this long to get to know all the ins and outs of whats involved.
Generally certain people seem good at certain things, whether they're artistic and therefore good at the posters, publicity type stuff. Others may have "contacts" and be great at getting great raffle prizes off people. We have a couple of mums that don't have a lot of free time but are fantastic face painters so will always do that at the fair.
Sometimes you feel a bit jaded, particularly if you seem to be working so hard and no one seems to notice or care or just take you for granted. When this happened I just took a back seat for a while and now feel refreshed and enthusiastic for joining in again.