Both are fairly broad, flexible concepts. The mafia subforum (particularly the Beginner's Mafia threads) has a fair amount of (fairly long-winded) info on what it is, but there's no real RTD explanations/tutorials anywhere from what I've seen.
Fundamentally, Mafia is a game about an informed majority ("town") versus an informed minority ("scum"). Town tries to figure out who's actually scum faking being town, scum tries to act like town. The game is split into a Day phase, where players talk, and then a Night phase, where players use their powers. When the Day phase ends, whoever has the most votes is lynched, meaning they're out of the game and their role (what powers they have, if any) and alignment (town, scum, neutral, etc) are revealed. Town outnumbers scum and thus can vote to lynch them if they figure out who they are, but have no direct way of knowing who's town and who's scum, and so frequently end up mislynching other town instead. Scum know who each other are and can kill a target at night, but are of course outnumbered in both votes and other power roles.
Different gametypes can alter just about anything up there. Some games feature a Cult in place of traditional scum, for instance, who the convert townies to scum rather than killing them. Other games feature occasional powers that can be used during the day, or pairs of townies who know each other's alignment and can communicate freely with each other.
RTDs are play-by-post RPGish things that use a d6 and a table rather than more traditional methods. A 1 means a critical failure that makes things worse, a 2 means normal failure, a 3 means partial success, a 4 means normal success, a 5 means critical success, and a 6 means an overshot that succeeds so well it has side effects.
RTDs vary even more than mafia games, however. Some people use different scales or die sizes, such as using a d8 or declaring a 4 to be partial success, and there's noticeable variance between GMs as to what exactly various levels of failure or overshot mean (whether a 1 makes you look foolish or blows of a limb, or how "good" a 6 generally is). Most games also have a lot of other features, like tracking hit points or equipment rules, that aren't universal to RTDs in general.