This is a pretty big subject, and reams of paper have been written about it. I'll try to give you some general information on this complicated and controversial subject.
A cam is to an engine as software is to a computer. Once all the right parts are there, the cam will "program" what kind of power it can make, and where in the rpm band it will make it.
Keep in mind that - like a computer - the cam must match the hardware in the engine; heads, pistons, carbs, etc. And also has to be matched to the transmission, final drive ratio, and vehicle. You can't install an X303 cam in a stock 5.0 and expect it to perform well. The parts just don't match.
The commonly listed specs are lift, duration, and lobe center. More lift and duration will increase air flow (and therefore power). But as lift and duration increase, the power band also increases. In other words, higher up in the rpm range.
For Ford, the B, E, and X cams have progressivly higher lift and duration numbers. Therefore making more power higher in the rpm range. Check the catalog for specific numbers.
Lifters are another story all together. The lifters ride on top of the cam lobe, and transmit movement upwards into the pushrods, eventually actuating the valves in time with cam rotation.
There are four type: solid/flat, hydraulic/flat, solid/roller, and hydraulic/roller. Solids run at high rpm's, require periodic adjustment, and are a little noisy. They are predominantly a race part. Some folks like to use them because they sound cool. Hydraulic lifters are self adjusting to a certain extent, are quiet, and are limited to about 6,000-6,5000 rpm. Stock engines have had hydraulic lifters since the early 60's (or earlier?).
Flat tappet lifters are a solid piece of steel on the bottom, and this solid steel rides on the cam lobe. Roller lifters actually have a small wheel on the bottom. Flat tappets are cheap, easy to install, and have been in use forever. Rollers are much more expensive. But they are also much better. The rolling wheel has almost zero frictional losses, and almost no cam lobe wear. Decreased friction will make more power.
And the lobes on a roller cam are shaped different than a flat tappet cam. The flat tappet cam is shaped kind of like a parabolic orbit, nearly a pointed elipse. The roller cam lobes are almost flat on top, like a mesa with sloping sides. This allows the valves to open and close much quicker, and keep the valve at it's full open position longer.
This is a really complicated subject, and I know I've been a somewhat long-winded. For more details, go to the web sites of cam makers, like Comp Cams. When it's time to pick a cam, call those people for advice. They will ask you a boatload of questions about your engine, your car, and how you drive. When they recommend a cam, follow that advice.
I also have a book called, "Ford Windsor Small Block Performance", from HP Books. ISBN 1-55788323-8. List price is $17.95. There's a whole chapter on camshafts and valvetrains.
