Champions with (Summoned) can't take wild nora fonts or contest those of your opponent, leave no nora globe when they are destroyed, and have no spell presence. Generally when the word Summon appears in a rune or ability (such as Summon Bat) they will be summons.
Note champions which are summoned can still attack and use abilities, so they can be pretty useful. One particularly strong counter to summons is Sever Summons, which destroys all summoned champions.