If you took a trading card game (TCG) like Magic: The Gathering, added some board wargame mechanics, and threw in an online player-matching server, you'd have something very much like SpiritWars.
As in Magic, you have a 'hand' of spells you can cast, with your hand replenished one card per turn. Some of the spells create power sources (think Magic lands), of differing colors; others create defenders who do not move, but can be used to block attacks of enemies nearby on more vulnerable creatures (think walls). Still others are units that can move and attack--think creatures.
Rather than having a line of cards facing your opponent, however, the game is played on a hex grid. There's fog of war, so initially you don't know where your opponent is, but somewhere out there is his castle--and to win, you must destroy it. Getting out some fast creatures to scout is a good idea.
Some creatures have special abilities, like the power to heal; and someof your spells also have particular effects (think instants and continuing spells). And, of course, terrain comes into play; each creature (and power source) is more effective in the terrain linked to its "color."
The net effect is to sustain the obsessive metagame of "deck construction," in which you're always trying to find the mix of spells that allows you to implement an interesting strategy (in this game it's "dungeon construction"), while adding an element of planning and strategy through the boardgame elements that's lacking in pure TCGs.
The interface is clean, but not perfectly transparent--remember that right-clicking almost anything will provide information about a unit, and you'll be better off. Expect to be confused the first time you play, and don't worry too much about it; a single game will probably suffice to get the hang of things.
In free-play mode, you have access to about 200 different spirits (cards), and the ability to customize your dungeon (deck). As a paying member, you have access to more like 600, and hundreds of different maps--and gain new spirits with each victory (and lose some with each loss). And there's a large enough community that you can pretty much rely on finding an opponent when you go online.
A Little History
SpiritWars premiered in 1998 on WON.net, Sierra's now defunct online game service, and when WON was purchased by the Flipside network and was merged into their casual game service, Randy Chase, the developer, decided to keep it going, running it himself. The game's enthusiastic fans followed him off the service, and have kept it going ever since. Now in version 3.0, it's been iterartively developed over the years until it now contains a veritable wealth of different 'spirits' and maps, and has become a highly polished, smooth-playing game. This is, of course, one of the advantages of this kind of online game; it gets better with age and polish.
You can play forever for free, if you wish, but it's likely that, if you find the game appealing, you'll want to subscribe in order to get access to more varied spirits and maps, and to compete in a meaningful way with other members of the SpiritWars community. (Who are friendly and pretty adult, by the way--none of your Xbox Live foul-mouthed teenies here, these are serious strategy gamers.) Membership deals range from $25/3 months to $65/a year. But you're never obliged to pay, if you don't wish to.