The "summoned" mechanism keyword first appeared in the Call of the Crystals set while the "enters play" one first appeared in the Dark Prophecies set.

Summons & Enters

"Enters play" is a broader term than "summon" ; all "summons" technically are also "enters play", but not all "enters play" are "summons".

By default:

• Is considered as "summoned" any card that is played from hand with resource cost paid to play the card.
  
• Is considered as "enters play" any card that is summoned or comes onto the field in response to an ability, from anywhere (deck, hand or graveyard) .
  
• Any non haste summoned ally is exhausted.
  

Some abilities with summoned :
Some abilities with enters play :

Returned to

"Returned to play" is a card that comes back to the field from the graveyard.
A "returned to play from graveyard" ally without being readied is exhausted.
"Returned to your hand" is a card that comes back to your hand from the graveyard.

Situational examples

1. Forgotten Horror
   • When summoned, its passive ability is triggered.
   • When returned in play from graveyard, its passive ability is triggered.
   • When revealed from deck and enters play, its passive ability is triggered (with Stardust Actualizer).
2. Lily Rosecult
   • When summoned, its passive ability is triggered.
   • When returned in play from graveyard, its passive ability is not triggered.