The process of finding the IP address is achieved by searching the DNS (Domain Name Servers) until a match on the domain name is found. This process is also known as DNS lookup, NSLOOKUP or (erroneously) IP lookup
The process of finding the host name (or domain name) from an IP address involves sending a message to the IP address and requesting the computer located at that IP address to return its name. Usually this will be the same as the domain name. However, many computers host many domains so the host name may be one of the domain names hosted or it could be something totally different.
There are some special IP addresses. 127.0.0.1 is always the IP address of every computer. No matter which computer you use, it will always have an IP address of 127.0.0.1 and a name of 'localhost'. In addition, a computer can have more than one IP address. In order to connect to other computers it will have an IP address that is known to other computers.