Public ip are the ip that can be accessed by every one ie very user has the access to this ip's.e.g yahoo.com google.com etc are the pubic ip's.
Private ip's are the ip that can not be accessed by every one ie they are execively owned by an organization only the user of that organisation has the access to this ip's.
Range of private IP address are
class A 10.0.0.1 to 10.255.255.254
class B 172.16.0.1 to 172.31.255.254
class C 192.0.0.1 to 192.255.255.254
excluding ips are public ip's.