Nowadays, the threats on online security become so common. Various types of threats make our data no longer safe. Passwords, encryption and other prevention method have been suggested, however, threats still exist to cause harm.
Mainly, threats have been divided into technical attacks and non-technical attacks. Technical attacks mean attacks perpetrated using software and systems knowledge or expertise. They are:
(1) Denial-of-service (Dos) attack
an attack on a web site in which an attacker uses specialized software to send a flood of data packets to the target computer with the aim of overloading its resources;
(2)Distributed denial-of-service (DDos) attack
A denial-of-service attack in which the hacker gains illegal administrative access to as many computers on the Internet as possible and uses these multiple computers to send a flood of data packets to the target computer.
(3)Malware

A generic term for malicious software. The severity of the viruses increased substatially, requiring much more time and money to recover. Malicious code take a variety of forms-both pure and hybrid. There are a lots of it such as virus, worm, macro virus or macro worm and trojan horse (rookit).
For non-technical attacks, they uses trickery to trick people into revealing sensitive information or performing actions that compromise the security of a network.

Another form of non-technical attack, is called social engineering. It is a type of attack that uses social pressures to trick computer user into compromising computer networks to which those individuals have access.

Other than that, phishing is to collect information through fake websites. An email may be sent to unsuspecting customers to trick them into providing or updating their information on the fake web sites. To safeguard against phishing, users should always go to the website directly rather than via links in emails.

In order to prevent both kind of the attacks, education and training needed to be provided in order to make explanation about the techniques used by hackers and others. The policies and procedures needed to be developed for securing confidential information.
References:
http://en.wikipedia.org/wiki/Timeline_of_computer_security_hacker_historyhttp://www.readwriteweb.com/archives/top_online_security_threats_for_2009.php