Bicycle Mechanics - Trigger shifter?

I'm looking to purchase an old Raleigh Superbe and the guy who I'm thinking about buying it from told me that the trigger shifter needs some work. Now, I'm rather new to bikes and am not sure what this means. In all honesty, I'm not even sure what a "trigger shifter" is. So I have a few questions.

1. What exactly is a trigger shifter, and what makes it different from any other kind of shifter?

2. Assuming the worst, if the shifter is shot, how easy of a problem is this to fix? How much would it cost (ballpark)? (Lets say I need to get a brand new shifter.)

3. If the shifter isn't shot, but only needs some work (as the man says), what would fixing this problem be like? and again how much do you think that would run me?

Need more info. Is it a Sturmey Archer 3 speed? If so, the trigger shifter will likely look something like this.
http://i56.tinypic.com/2h2o1uv.jpg
If so, it most likely does not need replacing, just cleaning and some lubrication. Shifting issues with these shifters usually have more to do with the cable than the shifter. A new cable and housing may be in order.
These shifters use a special cable, not a generic one, but they are readilly available, (I probably have a half dozen of them here.)

Yes, it is a three speed, and that is exactly what the shifter looks like.
Im hoping that its a problem with it not being clean, that would be very easy to fix.

If it is the cable however, is that something I can fix at home? Or would I need to take it to a bike shop?

Thanks so much for your help.

Cable replacement is pretty straight forward, but you will need to know how to set the shifting adjustment at the hub.
These instructions are better than Sturmey Archer's. I always set up 3 speeds this way.
http://www.sheldonbrown.com/sturmey-archer/adj.html
If you run into problems, I'm sure someone on here can help you through it.

In manufacture for the last ~75 years (http://genetics.mgh.harvard.edu/hanczyc/pdfs/satriggersjuly06.pdf), and still a current product (http://www.sturmey-archer.com/products/shifters/cid/3/id/12). If it's really, really knackered, a brand new replacement should be available from your LBS; if not, they're widely available from many 'net sources (example (http://aebike.com/product/sturmey-archer-s3s-3spd-classic-trigger-shifter-sku-ld2205-qc30.htm)) and not expensive.

The work .. clean and oil, perhaps replace cable and housing. The one on My Brompton faces up,
It got a wee bit sluggish , and a few drops of Oil did the trick.

Thanks for all the help guys, but i have one more question.

Whats the likelihood of the not the shifter being the problem, but the gears themselves?

That possibility is there, but issues with those hubs are quite often resolved without opening them up. They are oil lubricated, and sometimes a cleaning and lubing of the indicator spindle, or flushing the hub with kerosene and adding light oil (ATF would work, not gear oil and definitely not 3 in 1)will resolve issues caused by gunk in the works.
They can and do wear and break however, so I can't sit here and tell you all is going to be right with the world, just that there is a good chance it will.
They are repairable though, you just have to find a shop that knows their way around in there.

AW3 has only 1 set of planetary gears, they never disengage, but there are 2 sets of driver pawls
one for high and the other for low, as you pull the indicator chain
you switch the purpose the planetary gears from overdrive to reduction gear.
direct is the one in the middle , so neither 4/3 or 3/4.

all else fails every repair part is available.. something Sram And Shimano have come up short on.