"Send" bonuses are links that send an item to a friend/neighbor. In some cases, you also get something in return. Some games have limits on "send" links that differ from "get" links.
"Get" links are links that give you something that was offered by a friend/neighbor. Some games have limits on "get" links, and those differences are many. FrontierVille for instance limits any user to 30 "get" links, where CityVille limits each user to about 5 "get" links of each resource type.