Answer:
Synchro Monsters are the newest type of Monster Card. Their color frame is white and include "/Synchro" next to their Monster Type. They are included in the Extra Deck (formally known as the Fusion Deck) along with the Fusion Monsters (Note that with the release of Synchro Monsters, Konami has placed a limit of 15 cards in an Extra Deck. This means that you can only have a combined total of 15 Fusion and Synchro Monsters in your Extra Deck).
You must Synchro Summon a Synchro Monster to place it on the field. To Synchro Summon a monster, you must have one Tuner Monster and one or more non-Tuner Monsters face-up on the field. The combined Levels of the Tuner Monster and the other monsters you wish to use as Synchro Material Monsters must equal the Level of the Synchro Monster. Once these requirements are met, you can send the monsters to the Graveyard to Synchro Summon the appropriate Synchro Monster face-up onto your side of the field (this is treated as a Special Summon as well). It is important to note that the Tuner Monster and the other Synchro Material Monsters must be FACE-UP on the field. Almost always, you cannot use more than one Tuner Monster in the Synchro Summon (except in the case of XX-Saber Gottoms).
For more information regarding Synchro Monsters, click the first "Related Link" below.
For more help on how to Synchro Summon, click on the second "Related Link" below.