/effect command

effect <player|entity> <effect> [seconds] [amplifier] [hideParticles]
effect <player|entity> clear

► Give speed 2 for 60 seconds to all chickens and hide the effect particles.
/effect @e[type=Chicken] 1 60 1 true

► Give regeneration 1 for 30 seconds to all entities (includes players) except for chickens and hide the effect particles.
/effect @e[type=!Chicken] 10 30 0 true

► Give nausea for 30 seconds to all entities (includes players) except for chickens and hide the effect particles.
/effect @e[type=!Chicken] 9 30 0 true

► Give blindness for 45 seconds to the nearest 3 players and show the effect particles.
/effect @p[c=3] 15 45 0 false

► Clear all effects for all players and entities/mobs.
/effect @e clear

► Clear the speed 1 effect from all players.
/effect @a 1 0 0 true

Information on potion effects:
http://minecraft.gamepedia.com/Potion_Effects