What is live view (put simply)?

I'm thinking about getting the Nikon D3000, and I've looked at reviews that have said it's major letdown is that it doesn't have live view. I've tried to search what it means, but I can't find a straightforward answer.

Could somebody please simply explain to me what live view is?

Live view is the DSLR equivalent to using a P&S camera. You use the LCD instead of the viewfinder. Some people like it, some don't.

The only thing I use the LCD for is the menu functions and to check histograms.

Live view allows you to view what your sensors sees on the back LCD of the camera instead of looking through the hole at the top.

All live view is effectively is that the image can be composed and viewed on the LCD screen rather than the viewfinder (like a point and shoot)

It can be an advantage in things like macro photography, photojournalism (think of the press pack all with their cameras over their heads shooting blind effectively).
It also gives a larger view of the image than the viewfinder.
It is a disadvantage in things like sports where it is easier to follow the action with the camera to the eye.

An SLR has a mirror between the lens and the film/sensor. Normally this is to transmit the view which the film will be recording to the viewfinder. When the shutter is pressed the mirror goes up and the image that was seen in the viewfinder is recorded on film.

In cameras which did not have this then the view through the viewfinder was not exactly that which was recorded on film (paralex error) as the view was slightly different.

Personally I very very rarely use it and the disadvantages outway any advantage (shorter battery life, slower to compose etc)

Live view is when you can see what's going on via the lcd screen instead of using the viewfinder.

Personally I don't have much use for this feature - and it's no good anyway in bright sunshine. I wouldn't say not having it would be a "major let down".