What we call a day on Earth is measured by the sun. Starting from noon, when the sun is at the highest point in the sky, it takes 24 hours for the Earth to rotate such that the sun reaches its highest point in the sky again. We start a day at midnight and end at the next midnight, so the time is the same as measured from noon to noon.
The concept of a day is complicated by the fact that the Earth has also traveled part of the way around the sun while it rotated on its axis.
Beginning with the orientation in figure (a), Earth had rotated 360 degrees as compared to the faraway background as shown in figure (b), after one sidereal day. A sidereal day is measure by the stars.
In order for the same place on Earth to be pointed directly toward the sun (when the sun appears at it's highest point in the sky), Earth has to rotate more than 360 degrees, to account for its change in location along the orbital path. Earth must rotate almost one degree farther for a solar day than for a sidereal day. This means a solar day is actually longer than a sidereal day, by about four minutes.
A sidereal day is 23 hours, 56 minutes and 4.1 seconds long, while a solar day is 24 hours long.
The same kind of difference is apparent in the measurement of a sidereal month as opposed to a solar month.
Please check out this video explaining the difference between solar time and sidereal time, produced by the Cassiopeia Project.