Cameras can be simulated by moving objects across the scene in the oppose direction. The camera needs a position within the world which can be (0,0,-3.0f). The camera also needs a direction in which it's pointing at (needs a direction vector) and this can be found by normalising the difference between the position and target. The right vector represents the positive x-axis of camera space and this must be perpendicular to both an up vector and the direction vector (cross product). The vector pointing to positive y-axis can be found by again calculating the cross product between the x and z axis vectors.
The LookAt matrix looks at a given target and transforms all coordinates to that direction.
Why is Deltatime used so much and what is it? Deltatime is the time between subsequent frames. Some computers are faster then others so having a single constant for say speed would lead for faster movement for those with faster CPU's. By using deltatime the speed can be balanced out so the speed is the same on any PC.
These are 3 values which can represent any 3D rotation: pitch (vertical rotation angle), yaw (horizontal rotation angle), roll (3D rolling angle). Camera systems deal with pitch and yaw by converting them into a 3D vector (representing a direction vector) using trig. These are gathered from mouse movement. Last mouse positions are stored and current mouse movements are calculated (higher the horizontal/vertical movement the higher pitch and yaw).