In Aperture 1, the Browser had only two different modes, Grid view and List view. Aperture 2 adds a new mode called Filmstrip. List view is pretty distinct (it's a table of your images), but Grid and Filmstrip are easy to confuse. Let's look at the differences between the two.

The primary difference is that Filmstrip acts like a long, horizontal filmstrip with your images, and Grid acts like a very tall slide sheet of your images, with its width fixed to the window width. A quick way to tell which mode you're in is to see where your scrollbars are--horizontal scrollbars mean Filmstrip. To toggle between them, select the appropriate button in the segmented control with the Browser, or use the shortcut keys of ^F, ^G, and ^L to switch between Filmstrip, Grid, and List views respectively.

When in filmstrip mode, changing the height of the Browser changes the thumbnail size. In Grid mode (and List mode), there is a slider to adjust thumbnail size to the bottom right of the Browser, and changing the height of the Browser changes how many images you see at once.

As you have probably found by now, pressing "V" will toggle you through the different viewer modes (Viewer and Browser, Viewer only, Browser only). When you switch to the Browser only mode, the Filmstrip option goes away, and if you were in Filmstrip mode, Aperture switches you to Grid mode (it switches back if you go back to the Viewer and Browser mode). After all, if you're in Browser only mode, you want to see as many images at once as possible.

Lastly, all of the different features, from filtering to changing the sort order and criteria (by switching the popup), work exactly the same between these browser modes.