If you tap Photos you will see that to make it easier to navigate your photos, the Photos app organises your snaps by Years, Collections and Moments.
Moments It's likely that when you first tap on Photos you will see your recent shots, grouped in what Apple referrs to as Moments. Moments shows you the photos you took at a specific time. For example, on a trip to the seaside for a day out with the family. Any photos you take at that location, at that time will automatically be grouped into a Moment so you can find them and share them easily later on. (If this isn't the first view you see, just tap anywhere on the screen to 'zoom in' to that Collection.)
Collections If you want to look back further then tap on < Collections (top right), and you can see Collections of photos you have taken, usually grouped by location and date. For example, our iPhone is clever enough to keep photos taken in Switzerland separte from those taken in London and Surrey.
Years Similarly, tap < Years to view your photos by year. This is probably the fastest way to find your photos from Christmas 2011. Just tap at the end of 2011 to be taken to the photos from that time.
The great thing about the Photos app is it shows all your photos, not just those taken with the iPhone or iPad camera. So if you have shared images from your Mac to your iPhone, or from your iPad to your iPhone you can see them all in these different views.