The Prop 8 case is about whether California could legally amend their constitution to revoke the right to marry from gays and lesbians after it had been given to them by a state court in the summer of 2008. The argument that they can't is that the federal constitution prevents them from doing so. How does the constitution say that? Well, there are two ways to argue that: the broad way and the narrow way.
The broad way would be to say that it's straight up unconstitutional to prohibit lesbians and gays from marrying, ever. This could be based on either or both of two clauses of the Fourteenth Amendment which, crudely speaking, say (1) you can't discriminate in who gets treated better and worse by your laws, and (2) you can't deny people special "fundamental" rights, one of which is the right to marry (though of course there's the question whether the right to marriage includes the right to same-sex marriage). The trial court (lowest level) decided that both of these constitutional clauses meant that same-sex marriage was required by the federal Constitution.
But the appeals court, while agreeing with the trial court in the outcome (that CA had to let lesbians and gays marry) got there via the narrow way. It's a bit complicated, but basically there's a 1996 Supreme Court case (called Romer v. Evans) that said you can't pass an amendment to your state constitution to take away rights from lesbians and gays specifically--that's just plain mean. (Of course, they used the fancy word of "animus.") So the appeals court in the Prop 8 case (called Perry for short, by the way) said that since gays had the right to marry in CA (albeit for a short time) a state constitutional amendment taking that right away ran afoul of Romer, thus it's unconstitutional for CA to refuse to marry lesbians and gays, but only because they had already given them that right. The appeals court left for another day the question whether it was always constitutional to deny lesbians and gays the right to marry.
The conventional wisdom among legal experts is that the Court is highly unlikely to take the Prop 8 case. It's too CA-specific (because of the appeals court narrowing the reasoning), and Justice Kennedy (the author of Romer) would probably uphold the lower court's ruling. I agree: it's unlikely that the Supreme Court will take the case. If they don't, that means the appeals court ruling would stand and gay marriage would become legal in CA again.
The DOMA cases all stem from different individual facts, but they're all challenging Section 3 of that law, which says that as far as the federal government is concerned, only marriages between one man and one woman count as marriage--even if a lesbian or gay couple is legally married under a state's law. (NB: this law, passed in 1996, was the first time (I believe) that the federal government has decided which state marriages it will recognize. Usually it's just a question of whether a couples has a valid state marriage license; if so, the federal government doesn't ask any more questions before honoring it.) So if one partner dies, the other will have to pay federal estate taxes to inherit her property, for example. And there are lots of other federal benefits that are based on marriage (immigration privileges, not having to testify against your spouse in court, social security, healthcare, and still many more) and thus because of Section 3 of DOMA are unavailable to lesbian and gay married couples in Massachusetts and other states that allow for same-sex marriages. The cases all argue that discriminating against couples based on sexual orientation in this way violates the federal Constitution's guarantee of equal protection for everyone: they say there is no good reason to deny lesbian and gay couples those benefits.
The stakes in the DOMA cases are also lower than Perry's (potentially) broad reasoning. If the Court finds that Section 3 of DOMA is unconstitutional (as eight lower courts have done), that decision would not force any state to perform same-sex marriages, nor would it even force any state to recognize a same-sex marriage performed in another state (striking down Section 2 of DOMA would do the latter, but that's not at issue in this round of litigation). Ruling that Section 3 of DOMA is unconstitutional would just make the federal government treat all state marriages the same, regardless of whether the spouses are of opposite sexes or the same: a gay couple in Iowa would then receive all the federal benefits that a straight couple in Iowa does, but a gay couple in Virginia still couldn't get married or get any federal (or state) benefits based on marital status. So a relatively limited change, though obviously it would still be huge.
I think that Windsor and Gill, the two most prominent of the possible DOMA cases, won't be heard for technical reasons (it's not clear that the plaintiff is allowed to sue in this situation in the former, and Justice Kagan likely having to recuse herself in the latter, if you're interested) but that the Court very likely will hear one of the others (Golinksi or Pedersen) because there have been so many courts that have ruled that this major federal law (DOMA) is unconstitutional--it would be highly unusual for the Supreme Court not to have its say on the issue.
There ya go: more than you ever wanted to know about the same-sex marriage cases the Supreme Court might take up this term! Any questions? We should find out which (if any) of these cases they decide to hear tomorrow afternoon, or possibly as late as Monday. Exciting!
UPDATE 11/30 at 17:42: Nothing today, so we might hear Monday or it might even take until their conference next Friday. Boo!