Forbidden cards are essentially "banned" cards, while limited simply means that you can only put 1 in a deck, semi-limited is up to two in your deck only. Some cards in the OCG/TCG are either forbidden, or limited, and this is due to their ability to create an unbalanced experience for the game.
Unlike in the TCG/OCG where you can have multiple limited cards with different names in your deck, in Duel Links you can only have a total of 1 limited card in your deck. For example you have limited card A and limited card B, and you already put 1 card A in your deck, you cannot add card B to your deck even if they are not cards of the same name. This rule is also true for semi-limited cards in which you can only have a total of 2 semi-limited cards in your deck.
I was very happy with the Nerf then I decided to hit the PvP to see some new decks with new ideas and OMG why is cosmo brain a Rare card?
It's effect is ridiculous with protector,white stone, psychic ace and dragon spirt of white, a 3100 attack moster that allows slivers cry play and even if it get floodgated its 2450 defences is very strong, the OTK potential of this deck is ridiculous but the thing is you will likely lose if you go first, the game is sadly becoming a matter of coin flip! !
I just find people saying "That's not enough: limit mask change, limit anki, ban spellbook, perish vampires, bla bla" to extremely hilarious. Like wth do you want people to run so that you get satisfied?! PLZ TELL ME!!!!!