Well for the normal PC user,I dont need to say what a P2P software is.P2P stands for Peer To Peer and is used the world-over these days for File-Sharing.Let us see what P2P is all about.

Now, the services on the Internet are basically modelled on to basic complementary Architectural models:
1)CS(Client Server ) model
2)P2P model.

Client server models are used where centralised control and security are critical.
Whereas P2P model is used wherever the hosted files have not much security,control constraints and especially when the file size is too large.

As the IMC Technical Paper 05 on USENIX states:

Peer-to-peer (P2P) networks, where commodity personal computers form a cooperative network and share their resources (storage, CPU, bandwidth), have recently emerged as a solution to large scale content distribution without requiring major infrastructure investments. By capitalizing on the bandwidth of end-nodes, content providers that use peer-assisted solutions can benefit from a cost-effective distribution of bandwidth-intensive content to thousands of simultaneous users, both Internet-wide and in private networks.

As www.cse.cuhk.edu.hk states:

For a past few years,peer to peer (P2P) file sharing systems are generating tremendous amount of traffic on today’s Internet.This form of communication paradigm is reshaping the way new network applications are being designed

Words associated with P2P

What is a peer??

Well peer is the person using the P2P network and can be anyone who’s PC is connected with the P2P File Sharing system.

Leecher:

The peer who downloads the file

Seed :

The peer who provides download to the file

Free-rider :

The peer who’s interested in just downloading and wont provide downloads himself.

What is a Torrent??

Usually P2P-wares provide the facility of downloading the Torrents before the actual download.
Torrents usually comprise of data on the number of chunks,their size,checksum(error correction),and the file tracker.
Whenever you use Torrent file you send a request to the server which automatically returns data about number of peers available and their locations to the software.Thus you get connected to the network.

Working:

The P2P system works on the principle of ‘TIT for TAT’.Thus you may recieve a good response for downloads and get a good download speed only if you contribute.You need to be supplying the other peers with much content for download.Only then would you be able to get a good download speed.

The original file is usually split up into various small portions called CHUNKS. Chunks are usually of the size 32 to 256 Kbs.

1)Well the above demonstrated model clearly demonstrates how copy-right contents can be easily ‘Smuggled’ through peers.P2P has become a big threat to the multimedia industry which is getting badly hit with the Illegal sharing of contents like music,movies and games.
The biggest proof of this is that nowadays Games ,Movies are easily available to users just a day after their release

2)Can help in cheap and fast information distribution.

3)Help in communications through video(webcam) and voice(eg:Skype)