What is the difference between the x.509 V3 extensions Basic Constraints and Key Usage to create a CA certificate?

These two actions seem to do the same:

• using the Basic Constraints extension in a X.509 Certificate to signify that it is a CA certificate and
• using the Key Usage extension e.g. to signify that the public key can be used for certificate signining.

What is the difference between these extensions?
Служат ли они одной цели или дополняют друг друга?