What is the difference between IDRAC, IPMI, and ILO?

These are all forms of out-of-band management. IPMI is a standard. DRAC is a proprietary offering from Dell. ILO is the HP ProLiant variant. ILOM for Sun/Oracle. In some cases, you may hear the terms used interchangeably.

The proprietary lights-out management solutions provide more integration with the hardware and often time have nicer features (monitoring, logging, access) than a generic IPMI implementation.