Leica was one of the earliest entrants in the mirrorless camera race (2006), as they simply transitioned their existing M-mount film camera to digital with the M8.
The M series are a bit different than most later mirrorless cameras in that the M-mount Leicas use an optical rangefinder (manual focusing) and don't rely upon Live View on the rear LCD or an EVF to compose. Later variations of the M (9, M-E, Monochrom, 240) are full frame, while the original M8 is APS sensor size.
In 2014 Leica introduced a second mirrorless camera type based around an APS sensor, the T, which uses a new set of autofocus lenses (though it can use Leica M-mount lenses via an adapter).
In 2015 Leica introduced yet another mirrorless camera, the SL, once again based around a full frame sensor. The T (and now TL) and the SL share a lens mount, the L-mount, with TL lenses being crop-sensor lenses mostly to be used with the T, and SL lenses being full frame lenses that can be used with either camera.
Here is the full set of mirrorless cameras Leica has produced, with complete data for each:
Available Leica L (SL, TL) Mount Lenses