What is a synchro summon?

There is nothing inherent about a Synchro Monster that stops it being used as the Synchro Component for another Synchro Summon. So you could freely use Ally of Justice Catastor and a Lv 1 Tuner for...

No, a Synchro Summon is a kind of Special Summon.

Not quite. A Synchro Summon is a Special Summon. A Tribute Summon is the same as a Normal Summon, except that when you Tribute Summon you need to tribute monsters. Also you can Special Summon as many...

Synchro Summons in themselves are Special Summons, so in that regard, they can be special summoned. Some effects let you 'just' special summon a Synchro Monster, such as Starlight Road. The...

An accel synchro summon works when you tuner your synchro monsters with a synchro tuner monster, creating a more powerful monster. There is only one of these monsters (as of December 13, 2009) that...