Some spells leave a body on the field. Summon-type spells call a creature with its own power and toughness that persists turn after turn, attacking, blocking, and triggering abilities. A Frostbound Colossus shrugs off damage each upkeep; a Blazing Phoenix comes back from the ashes.
A summoned creature stays on the field until it is destroyed, so trading your creatures against the enemy's - and choosing when to attack the wizard directly instead - is a core part of combat. Meet every creature, its stats, abilities, and the spell that summons it in the Bestiary on the main site.