An entry in your DNS table (zone file) that maps each domain name (e.g. you.com) or subdomain (e.g. abc.you.com) to an IP Address.  In other words, the A record specifies the IP address to which the user would be sent for each domain name.  For example, you can have abc.you.com point to one IP address, and xyz.you.com point to a different IP address.