Em Am Dm G C F Bb Bdim
these are chords that are (with the exception of Bb) diatonic to the key, meaning they are chords that are made from notes in the key of C (the c scale notes).
for example Em is E minor, or an E minor triad and contains the notes E, G, B.
C is C major, C, E, G.
It is also common to assign numbers to these chords' root notes. So C is I (roman numeral 1), F is IV, G is V and are all major triads.
ii, iii, vi are all minor (D, E and A minor triads).
vii dim is a diminished chords.
Bb is a chord that is built on the flatted 7th degree of the scale, it is borrowed from another scale.
All major keys will have a I, ii, iii, IV, V, Vi, viio set of chords (note upper case for major, lower for minor, a little circle like a degree symbol for diminished) that are made from diatonic notes to the scale. this is why the roman numerals are useful. If a song has a progression of I, V, vi, IV you can play those chords in any key and the relationship between the chords will remain the same. you fingers will need to change what they are playing but the progressions functional harmony will remain the same. this is called transposing, or changing keys with out changing the relationship of the chords in context.