We live in the Milky Way Galaxy, which is above average in size, mass, and number of stars. Satellite galaxies are smaller in these three measures. They used to be independent, but they found themselves attracted to the gravity of the Milky Way and became its satellite. Galaxies are strongly affected by tidal effects and cannot orbit their hosts for as many orbits as planets can. Each such revolution can take hundreds of millions of years, and their ultimate fate is to be absorbed; their stars to become part of their master galaxy.
Some that are already close to the Milky Way and orbit at an angle can pierce through the disk and come out of the other side on their last few revolutions.
Until about a year ago, we knew about 59 satellite galaxies of the Milky Way, which contains 200 to 400 billion stars. The most massive is the Large Magellanic Cloud, which looks quite misformed. It used to be a pretty spiral galaxy, but now its interaction with the Milky Way and other satellite galaxies like the Small Magellanic Cloud has made it look wonky. It contains about 30 billion stars and might be 10 to 20% as massive as the Milky Way.
The least massive satellite galaxies can have millions of stars, but we still discover new ones. A recent research paper proposed that galaxies like ours should have up to hundreds of satellite galaxies and found an additional tens on top of 59 already known.
Some are difficult to spot because they are faint, and some we cannot see well because they are behind the bright central bulge and thick disk of our galaxy. The brightness of stars, their density, and the presence of dust make it difficult to see what’s behind. One of the objectives of the space mission of the European Space Agency, Gaia, was to find more satellite galaxies via peering through the central bulge and the disk, and it found some indeed.