Adware is usually connected with freely downloadable software.
For instance shareware sites will often give you a free download software containing ads within the code.
This is accepted policy and a legitimate alternative for people who don't want to pay for software.
Software including games, utilities and applications are freely distributed as freeware products.
Often freeware will block features and functions of the software until you pay to 'register it'.
There are a growing number of software developers who offer their product as 'sponsored freeware' until you pay to register and get the full version.
In general, most or even all features of the freeware will be enabled but you will be viewing sponsored advertisements whilst using the software.
The ads usually show in a small area of the software interface or even as a dreaded popup ad on your PC desktop.
When you close down the software, the ads will stop.
This solution allows customers to try out software before they choose to buy and disable the ads.
As an example of adware, the popular email program, Eudora.
You may buy Eudora or run the software in a sponsored mode.
In the sponsored mode Eudora will display ads in a window on the program and various sponsored toolbar links.
Eudora adware however is not malicious.
It doesn't for instance track your habits or provide various information about you to any third party.
So this type of adware is just showing random paid ads within the software program.
When you stop the program the ads will stop too.