What's the difference between IComparable & IEquatable interfaces?

IEquatable tests whether two objects are equal.

IComparable imposes a total ordering on the objects being compared.

For example, IEquatable would tell you that 5 is not equal to 7. IComparable would tell you that 5 comes before 7.

IEquatable<T> for equality.

IComparable<T> for ordering.

In addition to Greg D's answer:

You might implement IComparable without implementing IEquatable for a class where a partial ordering makes sense, and where you very definitely want the consumer to infer that just because CompareTo() returns zero, this does not imply that the objects are equal (for anything other than sorting purposes).