Mythril Ore is found in the Overworld below level 35. You need to use an Iron Pickaxe or better to mine this ore. Must be smelted to make a Mythril Ingot.
Mythril can be used to make mythril bars, tools, armor, weapons and a mythril furnace, which gives a chance to get 2 items per ore. Mythril can also be used to craft a bow and obviously, a block of mythril.
Mythril armor is between tin and adamantium and has uses in between. The weapon does as much as a diamond sword, but has a decreased durability. The speed of the tools in between tin and adamantium.
Most of this stuff can be changed in the configuration files.