The A ("address") record is the most basic record type in the DNS. When a browser needs to open example.com, the resolver asks for the A record of that name and receives one or more IPv4 addresses to connect to. A name can have several A records; clients then pick among the addresses, which is a simple way of spreading load or providing a fallback.
An A record only contains an address. It says nothing about which service runs there, and many unrelated names can point at the same address, which is how shared hosting and CDNs work. The IPv6 equivalent is the AAAA record, and a site that supports both protocols publishes both.
Each A record carries a TTL that tells resolvers how long they may cache the answer. If you plan to move a site to a new server, lowering the TTL a day or two in advance shortens the period during which some visitors are still sent to the old address.
Example
example.com. 300 IN A 192.0.2.10
www.example.com. 300 IN A 192.0.2.10