What's an IP rating?

The IP (Ingress Protection) rating standard is a system for classifying the degrees of protection provided by the enclosures of electrical equipment.  Developed by the European Committee for Electrotechnical Standardization (CENELEC), these standards are designed to numerically rate an electrical product on the level of protection its enclosure provides.  By assigning different number codes, the product's degree of protection can be identified quickly and easily.  In the code IP65, for example, IP identifies this standard, the 6 describes the level of protection from solid objects, and 5 describes the level of protection from liquids.

Value FIRST DIGIT
Protection against ingress of solids.
SECOND DIGIT
Protection against ingress of liquids.
0 No protection. No protection.
1 Protected against solid objects over 50mm, e.g. hands or large tools. Protected against vertically falling drops of water.
2 Protected against solid objects over 12mm, e.g. hands or large tools. Protected against direct sprays of water up to 15° from vertical.
3 Protected against solid objects over 2.5mm, e.g. wire or small tools. Protected against direct sprays of water up to 60° from vertical.
4 Protected against solid objects over 1.0mm, e.g. wires. Protected against water sprayed from any direction. Limited ingress permitted.
5 Limited protection against dust ingress (no harmful deposit). Protected against low pressure water jets from any direction. Limited ingress permitted.
6 Totally protected against dust ingress. Protected against high pressure water jets from any direction. Limited ingress permitted.
7 Protected against immersion between 15cm and 1M.
8 Protected against long periods of immersion under pressure.