A Tribute Summon is a form of Normal Summoning a Level 5 or higher Monster by Tributing Monster(s) you control.
By default, Level 5 and 6 Monsters require one Tribute, while Level 7 and higher Monsters require two Tributes. Tribute Summon counts toward the turn's Normal Summon/Set limit.
Negated Tribute Summon
Tributing a Monster for a Tribute Summon is part of a Summoning condition. Therefore, if the Tribute Summon is negated, the Tributed Monster(s) will not return to the field.
Not Tributing Monsters
If no Cards are Tributed to perform the Normal Summon of a Level 5 or higher Monster (as in the case of "Mausoleum of the Emperor"), the Summon is not a Tribute Summon, unless a Card specifies otherwise (such as "Tribute Burial").
Tribute Set is to Set a Level 5 or higher Monster. The tribute must still be made, but the Monster is not shown to the opponent.
Some Cards require three Tributes to be Normal Summoned, such as “Obelisk the Tormentor”. Other Cards, such as "Moisture Creature", can be summoned this way for a greater Effect. Only Monsters that directly state "3 Tributes" can be Tribute Summoned with this method. You cannot tribute more Monsters than necessary.
Tributing Spell and Trap Cards
Some Cards allow you to Tribute Spell or Trap Cards in place of Monsters, such as "Dinomight Knight, the True Dracofighter".