CCNA 50

The most basic type of NAT is the dynamic NAT. The dynamic NAT is a one-to-one NATting where one internal address directly translates to an external address for a given session. Dynamic NAT can only support as many hosts as there are addresses in the pool at a given time.
Dynamic NAT can actually be used to fix problems with overlapping addresses. Two organizations can be using 10.x.x.x networks. But when they communicate with each other, they would appear as 172.16 at one side and 172.17 at the other. Dynamic NAT is the least common form used.

The most common form of NAT is the NAT overload, which is commonly called PAT. This allows to overcome the shortage of IP addresses on the internet by translating all internal addresses into one single external addresses with different ports. It is rumored that Microsoft came up with the term "PAT". NAT Overload is actually the technically correct term.

PAT translates the internal IP to the external IP, keeping the same port number. If somehow two devices use the same port numbers to communicate, the later device would have its external port number incremented by 1.

For inside to outside, you'll need to use PAT. For outside to inside, you'll have to use static NATs. Static NAT can either be done for certain ports (e.g. port 80 goes to the web server, and port 25 goes to another server), or the entire IP address other than the already overloaded ports. You don't have to use the WAN interface address for NAT.