A Seyfert Galaxy is a group of galaxies whose existence was spotted by American astronomer Carl K. Seyfert hence the name. Their existence was noted in 1944. Their luminosity can change over time in matters of days and weeks. It is believed that the 1% of Spiral Galaxies are Seyfert Galaxies. The first Seyfert, BL Lacertae was thought of as a variable star before being identified as a Seyfert Galaxy.Britannica
Seyfert Galaxies fall into two main types, with additional subtypes devised by Daniel Osterbrook in 1981 : Georgia State Uni
A well known example of a Seyfert Galaxy is NGC 7742 in the constellation of Pegasus. As you can see, this particular one resembles an egg with a central yellow core. It is 74.2 Million Light Years away, a long distance but it is close compared to the nearest quasar which is about 600 million light years away.
The main differences between Quasars and Seyfert Galaxies are :-