PAT (Port Address Translation) : 

---PAT is used to convert Multiple source ip's into a [ single public ip / pool -multiple public ip's ] using Port Numbers.

In above example or practical we will do PAT on Router R1 which is END Router of LAN Having network of 10.0.0.0 255.0.0.0

R1(config)#int f0/0

R1(config-if)#ip nat inside ---- Inside Interface i.e Our LAN 
R1(config-if)#ex

R1(config)#int fa1/1

R1(config-if)#ip nat outside -- Outside Interface i.e WAN
R1(config-if)#ex

Now configure a access list to match the source traffic : 

R1(config)#access-list 1 permit 10.0.0.0 0.255.255.255    - Our LAN Network.

Now we have to do PAT : 

R1(config)#ip nat inside source list 1 interface f1/1 overload  -- all traffic from LAN get converted into a single ip of interface FA1/1.

To convert it into a pool of ip's we have to creata a pool : 

R1(config)#ip nat pool abc 12.0.0.3 12.0.0.5 netmask 255.0.0.0

R1(config)#ip nat inside source list 1 pool abc overload

TO VERIFY : 

show ip nat translation  --- Entry will be created when traffic will hit the R1 LAN Interface.

Rajan has already explained you people really well.

Here's a video tutorial for Step by Step Configuration guide on How to Configure PAT (Port Address Translation) using Cisco devices:

Well describe Rajan

Good one Rajan.