A Main Sequence Star is a star in the phase of burning Hydrogen into Helium, through the Proton-Proton Chain or the CNO Cycle, i.e., the Main Sequence. Such stars fit on a narrow diagonal region (roughly a line) on an H-R Diagram, the region/line also being referred to as the Main Sequence. For many stars, this is the majority of their Fusion lifetime, e.g., billions of years for a star such as the Sun.

(The term Main Sequence Star is also sometimes used in a general way for a star that has or will have a main sequence in its lifetime, even if it is before or after this phase, excluding, for example, Brown Dwarves.)

The term Dwarf Star actually refers to any star that is not a Giant Star, basically a main sequence star (though White Dwarves might be meant as well).