A Summon is the action of placing a Monster onto a Monster Zone. You can Summon a Monster either manually or with a Card Effect.

Always face-up
You always Summon Monsters face-up, unless you Special Summon with an Effect like "Morphing Jar 2", which states that the Monster is Special Summoned face-down.

You choose the Monster Zone
If you conduct a Summon, regardless the method, you will be the one who chooses which appropriate Monster Zone the Monster will be placed in (unless specified otherwise).

There are 10 different types of Summons:
Normal Summon
Tribute Summon
Flip Summon
Special Summon
Fusion Summon
Ritual Summon
Synchro Summon
XYZ Summon
Pendulum Summon
Link Summon

A Tribute Summon is a Normal Summon which requires a Tribute. Fusion, Ritual, Synchro, XYZ, Pendulum, and Link Summons are all Special Summons.