CORE RPG RULES
Iconoclast -- RPG -- Modifiers
Combat movement takes place at a heightened speed, since it's highly unlikely that anyone is going to be casually strolling along while they are being fired upon by machine guns. Thus, combat movement (your Speed value, in yards/meters per round) is the default type of movement, and applies no bonuses or penalties. Moving faster or slower than this, however, does change things a bit.
-40 Evasive Maneuvers
+40 Standing Still
Moving at normal walking speed (typically 1/2 your Speed) while in combat offers a +20 bonus to all targets concerning you. That is to say, you gain a +20 to your targets, but your opponents also gain a +20 to their targets when they're trying to hit you. Since you're moving slower, it's easier to aim at other people, but it's also easier for them to aim at you. Standing absolutely still during combat gives you a +40 on skill rolls, but as expected, also gives your opponents a +40 to hit you. Not usually a good idea.
Moving at greater than combat speed also affects you, as one might expect. Running away (or towards someone) in a more-or-less straight line (as when one is pursuing a fleeing suspect, or running down a long hallway) incurs a -20 penalty to all your targets, as well as the targets of those who are attempting to hit you. Running in an evasive way (dodging from side to side, rolling, and generally moving in an unpredictable way) incurs a -40 penalty to both actor and reactor.
Movement at speeds higher than normal walking speed results in Vitality loss; combat-style movement (roughly twice normal walking speed) results in a loss of 1 Vitality per round, and running results in a loss of 2 Vitality per round. See the section on Vitality loss and regain for more details.