See the chart below. The major scale for the Key of C is C, D, E, F, G, A, B. SO, the chords for the key of C are C, Dm, Em, F, G, Am and B diminished. The chart shows all chords in each major key. The chart can be a useful reference when playing jazz or blues chord progressions. For example, if someone tells you that the progression is a ii-V-I progression in Bb, you would know the chords referred to are Cm, F and Bb.
To better understand how chords relate to scales, see this discussion at musictheorysite.com.