There are many deck types in yugioh that identify what the deck's primary combos are.
Different Deck Types:
OTK/FTK - One Turn kill/First Turn Kill:
These decks mostly rely on special summoning monsters in swarms or with big enough numbers to take you out in one shot. Or the type of deck that makes big set up plays and activates a certain effect that cleans you out completly (Graveyard Burn).
These decks rely on nothing but speed, offence.
These decks are severely defenceless, if you can stop their big plays then you are in a good spot, but you have to becareful sometimes you cannot stop them.
Examples of these decks:
Cyber Stein OTK
Colossal Fighter OTK (Outdated and Ruling changed)
Dark Strike Fighter OTK
Exodia FTW (First Turn Win)
Cards to consider against these decks:
These cards will prevent them from OTKing you, with the exception of Shooting Quasar Dragon, but you should have used Effect Veiler to prevent them from using effects to synchro summon. Gorz/Tragodia/Battle Fader make sure you survive another turn even when you don't have veilers, and Maxx C allows you to draw into those cards above.
Themed Decks are the result of a specific Archtype/Starter/Structure Deck that was released and most of the cards that were in the decks were used.
These decks are balenced decks all around, some being stronger than others but have different strengths and weaknesses.
Here are some Themed Decks:
There is only one way to prepare against themed decks - Research
Once you research and play the decks for yourselves you will be able to see the strength and flaws of the deck and work with them.