Unlike computer viruses and worms, Trojans are not able to self-replicate.
How Trojans can impact you

Trojans are classified according to the type of actions that they can perform on your computer:

Some hackers make attempts on computers, servers or network systems just for the personal gratification. Others may feel that they need to prove something to their peers or friends, and hack something only for the challenge.

Hacking to steal

Another reason to hack a system is to steal information or money. A large portion of hacking attempts fall into this category. Banks and large companies are common targets for hacking jobs, but sometimes smaller companies or even a specific person's computer are targeted, as well.

Hacking to disrupt

There are also some hackers, including hacking groups; that target a company to disrupt business, create chaos and just be a nuisance. These groups often be trying to make a statement with their hacking, demonstrate security inadequacies, or to show general disapproval for the business itself. Examples of hacking groups that made headlines are Anonymous and LulzSec.
Hacking Techniques
Keylogger

Keylogger is a simple software that records the key sequence and strokes of your keyboard into a log file on your machine. These log files might even contain your personal email IDs and passwords. Keylogger is one of the main reasons why online banking sites give you an option to use their virtual keyboards.
Denial of Service (DoS\DDoS)

A Denial of Service attack is a hacking technique to take down a site or server by flooding that site or server with a lot of traffic that the server is unable to process all the requests in the real time and finally crashes down. For DDoS attacks, hackers often deploy botnets or zombie computers which have got the only work to flood your system with request packets.
What is a Back door?
A back door is a means of access to a computer program that bypasses security mechanisms. A programmer may sometimes install a back door so that the program can be accessed for troubleshooting or other purposes. However, attackers often use back doors that they detect or install themselves, as part of an exploit.

Hardware firewalls can be purchased as a stand-alone product but are also typically found in broadband routers, and should be considered an important part of your system and network set-up. Most hardware firewalls will have a minimum of four network ports to connect other computers, but for larger networks, business networking firewall solutions are available.
Software firewalls are installed on your computer (like any software) and you can customize it; allowing you some control over its function and protection features. A software firewall will protect your computer from outside attempts to control or gain access your computer.
Common Firewall Filtering Techniques

Firewalls are used to protect both home and corporate networks. A typical firewall program or hardware device filters all information coming through the Internet to your network or computer system. There are several types of firewall techniques that will prevent potentially harmful information from getting through:

In practice, many firewalls use two or more of these techniques in concert. A firewall is considered a first line of defense in protecting private information. For greater security, data can be encrypted.
Next Generation Firewall (NGFW)

A newer class of firewalls, next generation firewall - NGFW, filters network and Internet traffic based upon the applications or traffic types using specific ports. Next Generation Firewalls (NGFWs) blend the features of a standard firewall with quality of service (QoS) functionalities in order to provide smarter and deeper inspection.