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), CL, and the SL share a lens mount, the L-mount, with CL/TL lenses being crop-sensor lenses mostly to be used with the TL and CL, and SL lenses being full frame lenses that can be used with all L cameras.
Here is the full set of mirrorless cameras Leica has produced, with complete data for each:
Available Leica L (SL, TL) Mount Lenses