Adware is a type of malware which download advertisement content from internet and displays advertisements in the form of pop-ups, pop-unders etc. Once the Adware in installed on computer, they are not dependent on your browsers and they can display advertisements stand-alone. The pop-up blockers also cannot block these pop-ups. Adware is always an annoyance to the computer user.
Toolbars are available as plug-ins to browsers which provide additional functionality such as search forms or pop-up blockers. Examples of useful toolbars are Google Toolbar, Yahoo toolbar, Ask toolbar etc. There are malware toolbar plug-ins which are installed without the users consent and display advertisements and perform other nuisance activities.
Hijackers are another type of malware that take control of the behavior of your web browser like the home page, default search pages, toolbar etc. Hijackers redirect your browser to another URL if you mistype the URL of the website you want to visit. Hijackers can also prevent you from opening a particular web site. Hijackers are annoyance to the users who use the browser often.