A young child who has been bitten by a particular dog will become classically conditioned to the sight of that dog; that sight will evoke strong fear. If the child now also shows signs of fear to other dogs, merely because of their similarity to the dog that inflicted the bite, these responses would be examples of fear generalizing to the new (dog) stimuli. Later, the child may learn that these other dogs are friendly, so that they no longer evoke fear, although the original dog still does. This difference in response to similar stimuli would be an example of discrimination.
A generalization gradient is a graph showing the strength of the conditioned response as a function of some CS variable. Subjects are first conditioned to respond strongly to the CS (e.g., a 1000 Hz tone) by pairing it with a US. They are then tested for their response to other stimuli identical to the CS except for the values of one variable (e.g., tone frequency). Typically, subjects will respond most strongly to the original CS and less to the other test stimuli as they differ more and more from the CS. The result is a generalization gradient whose peak is located at the value of the original CS, with response strength falling off in either direction (e.g., higher or lower frequency tones).
Delay and trace conditioning both arrange for the CS to precede the US, so both are forms of a more general category called forward conditioning.