Raw and Smackdown are both franchises of the World Wrestling Entertainment or WWE. There are wrestlers that you usually see in Raw but not in Smackdown and vice versa. One obvious thing you would notice is the ropes used. In Raw, red ropes are used in the ring while blue ropes are used in Smackdown. Superstars John Cena, Randy Orton, and Dean Ambrose are exclusive to Smackdown while Brock Lesnar, Roman Reigns, and Seth Rollins can be seen in Raw.
The WWE World Heavyweight Championship and the Intercontinental Championship are on Smackdown; the WWE Universal Championship and WWE Tag Team Championship, on the other hand, are on Raw. You could always catch Raw during Mondays, and if you want to watch Smackdown, it is aired during Tuesdays.