"A" records (also known as host records) are the central records of DNS. These records link a domain, or subdomain, to an IP address. An A record tells a DNS server what specific IP address to map for a host name. It is the most common type of DNS record. An A record is typically used to direct your domain name, for examaple - www.yourdomainname.com - to a web server.
A records and IP addresses do not necessarily match on a one-to-one basis. Many A records correspond to a single IP address, where one machine can serve many web sites. Alternatively, a single A record may correspond to many IP addresses. This can facilitate fault tolerance and load distribution, and allows a site to move its physical location.