Synchro Monsters require Tuner Monsters to be Summoned. When the total Level of the Tuner monster and the other monsters on your field exactly matches the Synchro Monster's Level (no more or less), by sending them to the Graveyard, you can Summon the Synchro monster. Some Synchro Monsters require a specific "Tuner Monster" in order to be Synchro Summoned (such as "Junk Synchron" for "Junk Warrior") or monsters of a particular Type or Attribute as Material Monsters, but others do not. The Tuner Monster and the Non-Tuner Monster(s) must all be face-up on your side of the field to Synchro Summon
Because Synchro Monsters belong in the Extra Deck, they return there whenever they would be returned to the hand or Main Deck, like Fusion Monsters before. Also, like Fusion Monsters, they cannot be Special Summoned from the Graveyard if they were not properly Synchro Summoned first.
Synchro Monsters may be considered a more refined version of an older method of summoning monsters from the Fusion Deck known as Contact Fusion. Both Contact Fusion and Synchro Summoning simply require certain monsters to be on the field in order to summon the desired monster; the main difference is that Contact Fusion does not require the player to keep track of the number of levels of the monsters on the field, and the cards used for the summon are sent to the Graveyard.
Some players have begun to use the term 'Synchronize' or 'Tune' to describe the fusion of a Tuner Monster with non-tuner monsters immediately before performing a Synchro Summon and summoning a Synchro Monster.
There are currently no Aqua, Fish, Divine-Beast or Rock-Type Synchro Monsters in TCG or OCG. However, they have already been released in certain Archetypes of monsters including the Aliens, Iron Chain and Blackwing monsters.