The three C's  in the Diamond Guide are Color, Cut and Clarity... Combined, they  determine the fourth C... The Cost.

Color: Diamonds are rated from D (which is the whitest) to Z (Brown). The closer you get to D the higher the cost. Engagement Ring Diamonds are typically between between D and G in Color.

Cut: There are many cuts but the most fascinating cut is the Round Brilliant!....

Clarity: This reflects the purity of a Diamond.  Diamonds are rated by how pure they are or how many inclusions (flaws) they have.

F= Flawless or IF= Internally Flawless

VVS= Very Very Slightly Included

VS = Very Slightly Included (Also VS1 and VS2 are used)

SI = Slightly Included

I = Included (Typically avoided)

Typically, the minimum acceptable as a wedding diamond (unless you can afford more), would be SI or VS2.

Cost: A general order of magnitude of an average engagement ring is $3,000.