A ketone is a compound whose molecules contain a carbonyl group and have two other groups attached to the carbon atom of the carbonyl group. There are many molecules that belong to this classification, but the simplest one is acetone. A condensed structural formula for acetone looks like this.
In this formula, the C=O represents the carbonyl group, and the two CH3 groups satisfy the carbon atom's valence of 4. In other molecules that contain the carbonyl group, the C=O is still present, but the two CH3 groups are traded for other atoms or groups of atoms.
Sometimes we need to talk about the entire class of possible ketone molecules, and then we use a structural formula that looks like this.
In this picture, R and R' can stand for any hydrocarbon-containing group and as CH3-,C2H5-, etc.