Qucs - what is DC sweep?
SMIIW...

The term 'sweep' in this context refers to a change from one value to another.

So a \$V_{SWEEP}\$ of 3-8V would be a change from 3V to 8V over a predefined time.

A DC Sweep would therefore be a change in voltage of a DC source.  An AC sweep could be a change in an AC voltage over time, or more commonly, a change in AC frequency over time.

Any adjustable parameter can be the subject of a sweep - for instance turning a 100KΩ potentiometer from one extreme to the other would be a sweep of 0Ω-100KΩ

If you look at this picture:


(source: infn.it)  

You can see that \$V_{SWEEP}\$ goes from 0V to 1.5V, and the output (V2.1) is calculated for each point along that sweep as the output from the circuit if the input were what \$V_{SWEEP}\$ was at that time.