Some notable differences between a main sequence star and a red giant are its sizes and color. Red Giants tend to typically be, well, red. The output of a red giant is so much greater than that of a main sequence. A red giant is the last stage of a main sequence star's life. Another difference is what occurs inside the star. Main sequence stars fuse hydrogen together in order to keep gravity from imploding the star, while red giants run short of hydrogen and fuses helium.
