What is the difference between a.Equals(b) and a == b?

I am thinking if the diffrence is that

= = only compares the value of two strings but

a.Equals(b) by default compares the reference of both strings , but you could override this method to support value equality .

Am i correct or is there something else I should be aware of ?

Below isExtract from MSDN :Conversation on .NET

Q: Is there any difference between the DEFAULT Equals function (implemented in Object) and the =?

Anders Hejlsberg: For two reference types (classes), the default Equals() method is the same as ==. But for value types (structs), there is no == operator unless you provide an implementation in the value type.

What do you mean by"provide an implementation in the value type."