The netfilter software is capable of many different types of NAT. IP masquerade is one simple application of it.
It is possible, for example, to build NAT rules that translate only certain addresses or ranges of addresses and leave all others untouched, or to translate addresses into pools of addresses rather than just a single address, as masquerade does. You can in fact use the iptables command to generate NAT rules that map just about anything, with combinations of matches using any of the standard attributes, such as source address, destination address, protocol type, port number, etc.
Translating the source address of a packet is referred to as Source NAT, or SNAT, in iptables. Translating the destination address of a packet is known as Destination NAT, or DNAT. SNAT and DNAT are targets that you may use with the iptables command to build more sophisticated rules.