Refers to the number of switches or outlets in a single unit. For example, a 1 gang switch has one switch, a 2 gang has two, and so on. This applies to power sockets as well, where a 2 gang socket has two outlets in one fitting.
IP (International or Ingress Protection) defines a products level of resistance to dust and moisture. The first digit is for dust and the second digit is for moisture.
An IP65 rating offers a high resistance to moisture and is popular rating for outdoor lighting and electrical enclosures.
IP65 is usually specified for bathroom downlights but according to the Building Regulations, IP44 provides adequate protection for zones 1 & 2.