tactile = you feel a "bump" of resistance as you press down on a key. Usually there is a specific mechanism built into the key switch to deliberately produce this sensation as tactile "feedback".
click = you hear an audible "click" as you press down on a key (*before* it has hit bottom, usually between 1/3 and 2/3 of the way down). There is usually a specific mechanism built into the key switch in order to deliberately produce this audible "feedback".
clack = the key hits the bottom of the downstroke and therefore hits the bottom of the keyboard and "smacks" against it. Therefore this can potentially occur on any kind of switch. This sound occurs commonly on nearly any kind of keyboard, when the key is depressed fast or hard all the way down, and its volume is generally a function of how hard or fast you had hit the key, and so usually occurs on nearly any keyboard, rather than being a product of a deliberate mechanism. On some boards of course it is louder than others depending on the build and materials of the keyboard. People often mistake this for a built-in click. The volume of the clack however, it is true, can lend a different character to a keyboard's feel, depending on its characteristics.
linear = A keyswitch is linear when there is no noticeable tactile bump or hump of resistance on the downstroke. Instead the downstroke is "smooth". The force/resistance graph of the key shows a linear graph, therefore.
