If you look at a photo of Mars one thing that stands out is how scarred and battered it looks. There are thousands of craters covering the martian surface. Mars' proximity to the asteroid belt means it get bombarded by space rocks much more frequently than Earth (which is good news for us).
Occasionally the impact has enough energy to eject pieces of the planet into space. Mars' gravity is also only 38% of Earth's so materials reach escape velocity much easier. Every once in a while these ejected rocks cross paths with Earth and fall as meteorites. This is a very rare event as only around 124 martian meteorites have ever been discovered.
So, how do we know they are actually from Mars?
Meteorites from Mars exhibit precise elemental and isotopic compositions similar to rocks and also atmosphere gases analyzed by spacecraft on Mars, starting with the Viking lander in 1976. Those gases in the same concentrations were first found trapped within a meteorite known as Elephant Moraine 79001 and later in many other meteorites.
Viking 1. Credit: NASA
Analysis of the composition and the tiny pockets of gasses trapped within meteorites has confirmed at least 124 meteorites and proved beyond doubt that their parent body is in fact Mars.
NWA 6963 is one of the confirmed martian meteorites. For the official Meteoritical Bulletin: Click Here
For much more detailed information on the detective work that went into verifying these visitors from the Red Planet see here: Martian Meteorites