Speed is a very important stat for any Pokémon.
Speed is how fast any Pokémon is. However the speed of a moving Pokémon's sprite slows down while its getting near fainting. Its a very important stat for any Pokémon.
Speed and Evasiveness are different.
Speed only determines which Pokémon will go first in a turn. Evasiveness is different. Evasiveness of your Pokémon depends how easy it is for the Pokémon to dodge moves.
Speed-modifying moves.
Increase Speed:
Acupressure (By chance)
Agility
Ancient Power (By chance)
Automotize
Dragon Dance
Flame Charge
Gear Shift
Geomancy
Ominous Wind (By chance)
Quiver Dance
Rock Polish
Shell Smash
Silver Wind (By chance)
Decrease Speed:
Curse (Different for GHOST types)
Hammer Arm
V-Create
Decrease opponent's speed:
Bubble (By chance)
Bubblebeam (By chance)
Bulldoze
Constrict (By chance)
Cotton Spore
Electroweb
Glaciate
Icy Wind
Mud Shot
Rock Tomb
Scary Face
Secret Power (Real effect depends.)
Sticky Web
String Shot
Venom Drench
Items that modify speed.
Increase Speed:
Choice Scarf (Multiplies speed, but lets the user only use 1 move until called back.)
Quick Powder (Only works with Ditto.)
Salac Berry
Starf Berry (By chance. Can raise any stat at a time, its random though.)
X-Speed
Decrease Speed:
Iron Ball
Macho Brace
Power Anklet
Power Band
Power Belt
Power Bracer
Power Lens
Power Weight
Abilities that modify speed.
Increase Speed When Activated:
Chlorophyll
Motor Drive
Quick Feet
Speed Boost
Lower Speed When Activated:
Slow Start
Stall
That's a whole ton about speed!
There are also a few juices (available in only X/Y) at the Juice Shoppe.
Pink Juice (Raises speed EV. If speed EV is maxed, increases Happiness. Obtain by mixing 2 Pink berries.)
Perilous Soup (Removes all EVs. Obtain by mixing Kee Berry and Maranga Berry.)
Source 1 (Speed raising and lowering moves, abilities, items)
Source 2 (Speed EV modifying Juices and all other things about the Juice Shoppe)
EDIT: Quick Claw, Trick Room, Lagging Tail also modify Speed.