Are you gathering info for a motor build or just for general knowledge?
If its for a build, state your goals and you will most likely get more specific replies as to the amount of work that is appropriate for your application
generally speaking, you want to do things to the heads that allow for smoother and more efficient air flow through the head. Valve jobs, bowl blending, porting and polishing, are examples of the types of work that is commonly done. I wont define each, they are common terms you can search for more detail.
On top of that there are things to you can do the the mechanisms themselves, such as changing cams, valves , valve springs, retainers etc. In my opinion this is where you begin to get into the 'raceprepped' category, once you begin to exceed the factory RPM and operating temps, you will need to start upgrading the hardware to meet those needs. For example if you wanted a higher RPM application, springs with higher seat pressures are needed and then along with that you need valves designed to withstand those higher pressures. Lighter spring retainers don't hurt either.
here is an old ad from ASF machine, with some pics of a mild and more extreme example of porting polishing and a multiangle valve job.
there is some good reading in here by element tuning as to what they believe to be effective (scroll down for the head info)
more on porting