Summoning Sickness is a game term to describe the inability of creatures to attack or use activated abilities with the tap symbol in them on the turn they come into play. Although all permanents experience Summoning Sickness, only Creatures, Artifact Creatures, Land Creatures, and Enchantment Creatures (or Land, Artifacts or Enchantments that have become creatures) are affected by Summoning Sickness.
Note that summoning sickness only affects abilities with the image in the ability. If a card requires you to tap creatures, but does not tell you to tap the creature affected by summoning sickness, it does not prevent that effect. For example, Convoke and Selesnya Evangel's ability can be used with creatures with summoning sickness.