I am one. I go to meetings once a month or so between the full governing body meeting and the subcommittee I am on. In our school you are also given an area of responsibility - I am literacy governor. This involves liasing with the school's literacy co-ordinator and doing the occasional lesson observation. For some governors this is the extent of their involvement. There are lots of other things you can do though, I tend to volunteer for daytime stuff like sitting on interview panels or taking part in working parties as I am SAHM at the moment and most of the other governors are working. I also do the headteacher's performance management for which I have had training. Most of us at our school also do a bit of volunteer helping in the classroom - hearing readers and so on, to keep in touch with what is going on, but it is certainly not obligatory. Our head is keen to involve governors in the life of the school, not all schools are so welcoming apparently.
It is very rewarding, if you are considering going for it, I certainly recommend doing it.
Speak to your employer about time off for governor duties. Some companies are quite good about this.
I'm a parent governor and work full time. Our meetings full governors and committees meet once per term. Different schools work in different ways, have a chat with your chair of governors to find out what would be expected from you. Are their meetings during school hours or in the evenings? (Mine are in the evenings).
I hadn't planned on getting hugely involved but 3 years in I'm Vice Chair of Governors, Chair of Finance and ICT Governor.
As Tommy said you can more or less choose how involved you get.