Thanks both of you, I think I understand them more now; thank you especially Don for you simile: it made it clear for me to understand.
What about the second part of my question...
Which gaff deck would you say is the best?
Again, it's a subjective thing. One magician's favorite gaff might be another magician's LEAST favorite gaff.
Plus, there's a matter of definition here - because there are decks of gaff cards, then there are gaffed decks, and they're not the same thing.
A deck of gaff cards would be a boxed set of specific gaff cards, of which you'd use anywhere from one of them to a handful of them for a particular trick - a double-backer card would be a single gaff card, a three-and-a-half of clubs would be another, a series of four or five "Queen of Hearts" cards where the face fades to a lighter shade from card to card combined with a blank card to represent 100% faded, etc.
Gaff cards can be "packet tricks" where only a small number of cards are needed to perform the trick, or they can be meant for use when combined with a deck of standard cards having a matching back. For example, the "Ultragaff" deck from Ellusionist is designed specifically for use with a pack of red Bicycle Rider Backs, while the "Arcane" gaff deck is designed specifically for use only with their own Arcane brand of playing cards. Both contain packet trick cards that can be used in a subset independent of the full deck, but the bulk of the cards are meant to be used with matching standard deck.
Gaff cards, until recently, were more commonly sold either individually or in small sets. This was often the case because the seller, typically a magic shop or a fellow magician, either specifically ordered the cards to be custom-printed in this way or the cards were actually crafted by hand. A classic example of a hand-crafted gimmick might be a card that has two different "half-faces" on the same card - like the top half of a Queen of Hearts and the bottom half of a King of Spades; an old-school magician might make such a card by splitting the face from the back of one of each of these cards, precisely cutting the faces in half with a razor blade or an Exacto knife, then recombining the mismatched halves and regluing the two half-faces and the card back together, thus creating two gimmicked/gaffed cards. Many gaffs, such as those created by Kevin Reylek for his "Cannibal" trick, are not only made by hand, but the cards themselves are "consumed"/used up over the course of the trick, thus can only be used once - though often in such cases, the consumed gaff can now be handed out as a unique souvenir for the spectator.
A gaffed deck would be a trick deck - an entire deck meant to be used for performing a trick or category of tricks. The cards in it can appear perfectly normal or can have a specially-altered appearance, but the combination of the entire deck of cards works in concert for performing the trick. There may also be a special gimmick concealed in the box or among the cards, used for executing the trick. A marked deck, an "Invisible" deck, a stripper deck, a "Rising Card" deck, a "Brainwave" deck, a "Svengali" deck, a "Gambler's" deck, a "Mirage" deck - these are all examples of gaffed decks.
In many cases, the cards can't be passed off as a standard deck because they don't look like a standard deck - for example, the "Rising Card" deck could never be used to deal a few hands of poker because of the Rising Card gimmick concealed within, nor could a "Svengali" deck, because every other card is identical to each other and cut shorter than the rest of the deck. In other cases, the cards could be used in a standard card game, but would allow for a player "in the know" to use them to his or her advantage in the game - for example, using a marked deck when playing a few hands of poker will give the players who know it's a marked deck and how to read the marks a decided advantage over those who don't know, and a player using a stripper deck to deal some hands of a card game could set the deck up to allow a specific player to receive specific cards chosen by the dealer, granting an advantage.
It's really less a matter of choosing the "best" gaffed deck or deck of gaff cards, but choosing the ones needed for the tricks you want to perform. I wouldn't buy a Svengali deck for a gambling demonstration, but I might buy a stripper deck or a marked deck (or a deck that combines the features of both, like a Gambler's deck). I wouldn't buy an entire pack of gaffed cards if all I wanted was to perform a trick using two three-and-a-half of clubs cards or if I just needed the special gaffs for "MacDonald's Aces" or a unique version of a three-card monte routine.
Having said that, if I'm in the market for gaffed cards, buying a full deck of them tends to be more cost-effective than purchasing individual gaff cards or packet tricks, and will often come with an instructional video either included or as a separate purchase. A single gaff card will typically come with either no instruction (because it's thought to be so common or simple as to require no instruction) or it might come with a simple trick or two printed on a piece of paper packaged with the card.
Individual gaff cards, whether purchased as individual cards, small sets of cards or complete decks, often require a certain amount of sleight-of-hand skill to be used effectively - the ability to force a card, make a card "disappear", switch decks without the spectator noticing, etc. They often require a level of skill slightly higher than the typical beginner, and sometimes the instructional videos will demonstrate how to perform the tricks for the cards - but not necessarily the basic techniques of card forcing, deck switching, etc. that are needed to use them effectively, or if they do include such instruction, it's often of a more basic level. A beginning magician would be well-advised to supplement a purchase of gaff cards or a deck of gaff cards with some videos or books on sleight of hand.