1) It's TBS, like Titi said.
2) The basic premise of the game is the same as for Civilization: replay human history, from the stone age (4000 BC) to the 'near' future (2300 AD). It's combat is modern in the sense that Titi explained: you actually fight with combined arms, rather than one unit at a time (besides ranged attacks you can also flank or bombard enemies).
It's modern age is essentially not all that different than that of Civilization: there are Tanks, Cruise Missiles, Stealth Fighters, etc. There is one cool feature that Civ doesn't have: Nuclear MAD (Mutually Assured Destruction), though I'm not sure how often people actually use it (I'm not a big fan of the modern age myself), so you can replay the Cold War if you want. There is also unconventional warfare: you can set up franchises or use advertising or laywers to undermine an opponents economy. Of course there are also other unconventional other units, most notable for the modern age are the units for biological/chemical warfare. But there are also units for espionage and religion, though those are not exactly (or exclusively) modern.
The game does differ from Civilization in the sense that it goes further into the future: after the Modern Age there are two more ages and a number of interesting futuristic concepts are introduced: undersea cities and various kinds of undesea military units, and also nanite and eco-weapons and units such as hover infantry and tanks. CtP1 also has a 'space layer' where you can build space cities and fight wars with space fighters and bombers (and to bombard/attack earth cities from space), but this was removed in CtP2.
But the most important thing about Call to Power is the mods: you can change just about everything you want about the game and the fans have used this opportunity to do just that. About half a dozen to a dozen major mods exist for the game that offer very different gameplay than the original game: some focus one a specific time period (such as the modern age), some improve all aspects and time periods of the game, others just the AI, etc. You can find all these mods and an active playing & modding community at Apolyton:
http://apolyton.net/
3) I got my copy for free because the guys who made the game think I'm a nice guy (and I was an alpha/beta tester for the game)
But if you want to buy a copy now, you shouldn't have to pay much more than that. If there are plenty of game stores with a good assortment where you live, you should be able to pick it up for no more then /$5. If you have less choice you may end up paying twice that much, but certainly not more.
)
