Normally when you use Google Street View, you'll see the most recent available image captured by the Google Street View cameras. Google has now added an option where you can see older Street View imagery, dating back as far as 2007, in some locations.
The level of detail you'll see varies depending on the country — Street View didn't reach Australia until 2008 and most places have only ever been filmed once, so there won't be too many to see around here.
If there are other images available, you'll see a clock in the upper-left hand corner next to the date of the most recent Street View image. Click on that and a slider will be displayed letting you select images from other dates.
Go back in time with Street View [Official Google Blog]