Mythos is a free MMORPG (Massively Multiplayer Online Role Playing Game). In other words, you will join an open fantasy world with other players to complete missions with magic and teamwork.
Set on the planet Uld, which flickers in and out of the influence of the gods. This has resulted in a dark ages, with the planet torn apart by a battle between the forces of good and evil. So far so typical, and Mythos is a fairly standard fantasy game.
You begin by creating your Mythos character; choosing a class and appearance and so on. Then you join a server and start the game. As is standard for MMORPGs, you then embark on tutorial missions to learn the ropes of the game.
Gradually you gain experience by completing missions. The more experience you get, the more versatile your weapons and commands become. This encourages you to grow quickly.
For hardcore gamers, this will all sound very familiar, and Mythos certainly looks very recognizable to anyone who's seen World of Warcraft. The major difference with Mythos is there is no monthly fee to play as with WoW.
If you're curious about online fantasy role playing, but are put off by WoW's fees, you could do worse than trying out Mythos, although it is a pretty generic MMORPG over all.