IPv6 Versus IPv4 - what is the difference?

1. Simplified header format. IPv6 has a fixed length header, which does not include most of the options an IPv4 header can include. Even though the IPv6 header contains two 128 bit addresses (for the source and the destination IP address) the whole header has a fixed length of 40 bytes only which allows for faster processing. Options are dealt with in the extension headers, which are only inserted after the IPv6 header and only if needed. So for instance if a packet needs to be fragmented, the fragmentation header info is inserted after the IPv6 header. The basic set of extension headers is defined in RFC 2460.
2. Address extended to 128 bits. This allows for hierarchical structure of the address space and provides enough addresses for almost every 'grain of sand' on the earth. This is most important for security reasons and for more new devices/services that will need multiple IP addresses and/or permanent connectivity. 
3.  A lot of the new IPv6 functionality is pre-built into ICMPv6 such as neighbor device discovery, auto-configuration, multicast listener discovery, and path MTU discovery.
4. Enhance Security and QoS (quality of service) features.
   
   The numbers change is based on the version of internet protocol, ie: IPv4 means Internet Protocol version 4, and likewise IPv6 means Internet Protocol version 6.
   
   

IPv4 is a 32 bits IP address that we have used, and examples are 192.168.8.1, 10.3.4.5 or other 32 bits IP addresses. IPv4 can support up to 2³² addresses, however the 32 bits IPv4 addresses are almost used up so IPv6 was developed as a replacement.
IPv6 is a 128 bits IP address which can support up to 2¹²⁸ addresses to fulfill unlimited future needs with better security and network related features.
Here are some examples of IPv6 addresses:
1050:0:0:0:5:600:300c:326b
ff06::c3
0:0:0:0:0:0:192.1.56.10

The most important difference is that it has a larger address space. IPv6 uses 128 bits, instead of the 32 bits used in an IPv4 address. There are also some changes in the header format, and some additional options, like built-in security options. However, these can be added to IPv4 through additional protocols, so this part is really no big deal.