During your turn you may have as many cards in hand as possible, there is no restriction for this.

Before allowing the opponent to take their turn, you must have six or fewer cards in hand. If you have more than this amount than you must discard until you have six, this is according to the current rule book.

There are exceptions if a card allows you to have more than six in hand (like Infinite Cards).

Yep..can be found in basic rulebooks...

Until the End Phase of your turn, you can have as many cards as you want in your hand and, normally, if you have over 6 cards, you must discard until you are holding 6. Also, you only have to do the discarding during your OWN End Phase. If it is the opponent's turn and you end up having more than 6 cards in your hand, you DO NOT have to discard during your opponent's End Phase. Only your opponent does if he/she has over 6 cards.

Keep in mind, though, that this will NOT set off things that activate when "discarded by a card effect" like Dark World monsters since it is game mechanics discarding them and not card effects. However, effects that activate "when sent to the Graveyard" or something will still activate.

Infinite Cards, Heiroglyph Lithograph and Enervating Mist are some cards that can change this.