DSLR cameras have a coupled optical viewfinder that uses a pentamirror/pentaprism and a reflex mirror that allows the user to see the scene through the lens. They have all the controls needed to take a picture allowing the user to modify exposure value exactly to his liking. They also have relatively large digital sensors, the part that actually captures light so more details can be recorded in the picture. Finally, all dSLR's have an interchangeable lens system allowing the user to use the best lens possible for the job at hand.
High zoom or bridge cameras are souped up point-and-shoot cameras that have very little control and very small sensors. They however have the longest zoom ranges in the whole digital camera world. This is possible because of that small sensor inside them making high zooms relatively cheap and simpler to make. Then again, it is the shape of the bridge which is simply a miniaturized dSLR, that allows all that high zoom to fall into place. The lens of a bridge camera however is fixed and can not be replaced.
The a57 is an SLT, not a dSLR. It doesn't have a pentaprism/pentamirror and it's mirror is fixed for autofocus purposes only, not for optical viewing. The small viewfinder is electronic, not optical. The translucent mirror deflects part of the image upwards towards the autofocus mechanism of the camera allowing for faster focusing speeds. This is crucial when you shoot a lot of action and video. The technology however makes the camera somewhat weaker in low light shooting.