Q. How does parrying work?
A. Pressing the analog stick forward, towards the enemy (toward in relation to the characters, not the camera) and light/wide attack (X on 360 or Square on PS3) will cause Raiden to parry. You can do this move at any time that there is an incoming attack from said enemy, and the window for doing it is actually the entirety of said enemies attack animation. For example, the default enemies do a little spin and jump and attack you, as soon as they start said spin, you can start the parry, and Riaden will hold his sword out the entire time.
If you in fact, parry perfectly, that is, parry so that the sword comes out and hits the oncoming attack as a parry (and it doesn't look like a blocK) then Raiden will slash forward on his own, often causing enough damage or stun to allow for an execution move.
Q. Is there a dodge?
A. Yes. It can be performed by hitting X+A(360) or Square + Triangle (PS3) and a direction. If you are pushing forward, Raiden will do a jumping slash that will move him forward. The other directions will have him slash and hop into that direction, either back, or to either side.
Q. How about Lock-on?
A. RB/R2 Will lock on to your opponent. Your currently targeted or nearest enemy always has a white target symbol on them. Hitting this button will make it turn orange and force the camera to keep them on screen regardless of how you move around the stage. To change targets, de-lock by hitting the button, move to a new enemy till they have the white marker, and hit the button again. It's particularly useful against the Gekkos and Bladewolf, in fact, Bladewolf starts the battle locked on, and you only lose it when he brings in more enemies.