Gift: a present.
Is a wedding gift expected? Yes - everyone that receives an invitation should send a gift. (exceptions could be business associates or people you haven't seen in years)
How much should guests spend on gifts? Although giving a gift is a social obligation, the choice of the gift is based on the giver's affection and relationship to the couple as well as the financial capabilities. Family members usually spend a little more on the couple. You DON'T have to spend the equivalent of the per person amount the host spent for you at the reception.
When are gifts sent? Traditionally they are sent as soon as the invitation arrives. They should be delivered as close to the wedding as possible. There is a myth that says you have up to a year - the sooner the better but remember its never too late. Gifts should be sent to the couples home
(don't bring a gift to the wedding, the couple already has enough to deal with that day.) Envelopes of cash or checks can be brought that day but should be placed on a table set up for this, preferably in a box with someone looking after it.
Is record-keeping really necessary? Yes - absolutely. Keeping a record helps you associate specific gifts with the givers so you can send the appropriate thank you and mention it next time you see that person. It is also a good idea to keep a record in case you need to reference it when one of your guests marries.
What happens if the wedding is canceled? The gifts must be returned, all of them. If it is too difficult to return in person they can be mailed. You don't have to return monogrammed items.
adapted from Emily Post's Wedding Etiquette