IP (Internet Protocol) Address is a unique identifier that electronic devices and computers use in order to communicate with each other on a computer network.
An IP address is written as four numbers separated by periods. Each number (byte) can be between 0 and 255. Example of IP address: 254.123.0.33
There are five classes of available IP ranges: Class A, Class B, Class C, Class D and Class E, but only classes A, B and C are commonly used.
Class A - (The first byte is between 1 and 127) supports 16 million hosts on each of 126 networks
Class B - (The first byte is between 128 and 191) supports 65,000 hosts on each of 16,000 networks
Class C - (The first byte is between 192 and 223) supports 254 hosts on each of 2 million networks

Because IP addresses are hard to remember. The DNS makes it easier by allowing a domain name to be used instead of the IP address.
DNS (Domain Name System) is a distributed database that translates a computer's domain name into an IP address.
For example, it is much easier to remember www.google.com instead of the IP address 216.239.38.10
It is a "mnemonic" name that makes IP addresses easier to remember.