1. Relative cell reference: A relative cell reference in a formula changes when the formula is copied to another cell. For example, if a formula refers to cell A1 and is copied to cell B1, the reference will automatically change to B1.
2. Absolute cell reference: An absolute cell reference in a formula does not change when the formula is copied to another cell. It is denoted by adding a dollar sign ($) before the column letter and row number. For example, $A$1 will always refer to cell A1, regardless of where the formula is copied.
3. Mixed cell reference: A mixed cell reference contains a combination of relative and absolute references. For example, $A1 will keep the column fixed (A) but allow the row number to change, while A$1 will keep the row fixed (1) but allow the column letter to change.