The general rule is that you can only "Normal Summon or Set 1 monster per turn". Usually when people "play a monster", this is what they mean. Tribute Summons count as Normal Summons, so you cannot Normal Summon a monster and Tribute Summon in the same turn. Special Summons can be done as often as you like during your turn, as long as a card effect is causing the Special Summon.
During your turn, you can:
- Perform any number of Special Summons, as long as a card effect allows this.
- Perform a total of ONE (1) of either: Normal Summon (Level 4 or lower), Tribute Summon (Level 5 or higher), OR Set (but not Sets caused by card effects, e.g. a Special Summon face-down, or Book of Moon).
- Flip Summon any number of your own monsters, as long as the monster was already face-down at the start of your turn, or control of the monster changed this turn.