Movement speed is how fast a hero is travelling.
Base movement speed
The base movement speed when moving forwards is 5.5 meters per second, except for Tracer and Genji who move forwards at 6 meters per second. When moving directly backwards, the base speed is 90% of the forward speed (4.95 for most heroes; 5.4 for Tracer and Genji). When moving back diagonally, the base speed is of the forward speed (5.133 for most heroes; 5.6 for Tracer and Genji).
While crouching, the base movement speed is 3 meters per second for all heroes, regardless of direction.
If Wrecking Ball is in his roll mode, his base movement speed is 10 meters per second, regardless of direction.
There are three categories to speed modifiers: speed buffs, slow effects, and movement penalties. Each category stacks with themselves and are added together before being applied to the current base movement speed. However, self-inflicted movement penalties and enemy slow effects do not stack and the greater of the two overrides the other.
Suppose Reinhardt and Lúcio are on a team and Symmetra is on the other. Reinhardt walks forward in Lúcio's speed aura (+25% buff). His speed is of his base speed. Symmetra places down one turret (−20% slow). Reinhardt's speed is now of his base speed. Reinhardt uses his barrier (−30% penalty). His speed is now of his base speed because Barrier Field's movement penalty overrides the turret's slow. Symmetra places a second turret (−20% ⟶ −40% slow). Reinhardt's speed is now of his base speed, regardless of if he uses his barrier or not, because the two turrets' slow overrides Barrier Field's movement penalty. Symmetra places a third turret (−20% ⟶ −60% slow) and Lúcio amps up his speed song (+25% ⟶ +60% buff). Reinhardt's final movement speed is his base speed.
The only other case where one speed modifier overrides another is with D.Va's Boosters (+118% speed buff) and Fusion Cannons (−40% movement penalty): If D.Va is using Boosters, her Fusion Cannons' movement penalty no longer applies.