In a broad sense, hacking is an act by which someone gains access to a computer system or network without any authorisation to do so. Such unauthorised entry may or may not be used to harm the system.
The explosive growth of the Internet has brought many good things like ecommerce, online information distribution, collaborative computing and e-mail.
As with most technological advances, there is also a dark side: criminal hackers. Hacking is getting more sophisticated and, in many cases, a lot nastier. And it is chipping away at the ability of the government, the military, and the business community to protect proprietary information and preserve individual privacy. Organisations are afraid that some hacker will break into their Web server and replace their logo with pornography, read their e-mail, steal their credit card number from an online shopping site, or implant software that will secretly transmit their organisation's secrets to the open Internet.
Techniques adopted by hackers to gather information about systems are port scanning, sniffing and social engineering. Port scanning automatically detects security weaknesses in servers either locally or remotely. Sniffer is a piece of hardware or Software, which grabs all information tranversing Social engineering is an act by which valuable information about the network, passwords, access restrictions and user accounts are gathered from unsuspecting people.
A hacker could use the information thus collected to launch Denial of Service attacks, spoofing some ones source IP address, cracking passwords, lauching data attacks and packet fragmentation attacks. This is what is called blackhat or criminal hacking.
There is also good side to hacking in the form of whitehat or ethical hackers. They explore and experiment to evaluate target systems security and report back to the owners with the vulnerabilities found and also provide instructions to remedy them. These ethical hackers employ the same tools and techniques as the criminal hackers, but they neither damage the target systems nor steal information. Companies use ethical hackers to hackproof the security of their networks, ecommerce products or security products. Besides having to know the techniques of the criminal hackers, ethical hackers need to know how to detect their activities and also how to stop them.