Do you think that you are safe by using some smart antivirus tools to protect your 

computer from backdoors like remote administrator tools or from keyloggers, it is

good to use smart antivirus and firewall but you must consider sniffing as a security

 issue. There are so many tools available for various operating system including

 windows and Linux, we will talk about the tools later on but firstly there is a need to

 describe sniffing.

Network sniffing tools and packet analyser tools are generally the same thing, the main purpose of sniffing is to analyse the incoming and outgoing packets, keep in mind when you connect to a web server or to any network (computer) you send your data via wired connection or wireless connection in the form of packets, your data goes in the form of packets.

A packet contains the source (Sender) and the destination (Receiver) IP and MAC address and it also contains the data that are going to be sent. So just imagine if someone gets this packet then he/she can easily know what you have sent, an attacker can easily sniff your confidential information (credit card, paypal, passwords etc), so sniffing is the important attack that should be consider as a security measure.

Large organisation and web server uses secure connection for transmission, for example paypal uses https:// instead of http:// so its means your data goes in the form of encrypted data. If you are using SSL it does not mean that you are secure ever, there are ways to crack the secure channel.

All the thing that are discussed above shows that sniffing is not a good thing, however it is not true the Positive usage of sniffer is also its regular usage, which aims to maintain network and system working normally.

Capturing packets
Recording and analysing traffic
Decrypting packets and displaying in clear text
Converting data to readable format
Showing relevant information like IP, protocol, host or server name and so on.

While the negative usage is open and clear:

Catching password, which is the main reason for most illegal uses of sniffing tool
Capturing special and private information of transactions, like username, credit ID, account, and password
Recording email or instant message and resuming its content

The famous and the most widely used tools for sniffing purposes are:

Wireshark
Kismet
Netstumber
Kismac
Ettercap
Tcpdump
Cain and Abel
Etc