A satellite galaxy is a small galaxy, often a dwarf galaxy, orbiting a larger galaxy. A number of galaxies in the Local Group are satellites of the Milky Way or Andromeda. Some of the 30+ known satellites of the Milky Way:
Among Andromeda's satellites:
The exact number of satellite galaxies of the Milky Way and Andromeda are unclear as they are still being discovered, there is often discussion as to what constitutes a galaxy versus a globular cluster, and knowing whether it is orbiting depends upon its velocity, and such velocity determination is non-trivial and undergoes corrections over time.
Satellites in the Local Group are still being discovered and dwarf galaxies are even harder to identify in more distant groups and clusters.