The way you can tell them apart from their greater egret is the details around the eye. Great egrets have yellow extending above the eye, where lesser egrets have yellow extending beyond the underside of the eye.
These birds are very similar to their bigger variant species, the biggest difference is the size. They are much smaller. In breeding plumage, their legs and bill will turn red, and they lay anywhere between 1 to 5 eggs a season.
The photo itself, is of an egret gently gliding into land in the wetlands and begin hunting.