FeedItem
FeedItem represents an entry in the feed, such as changes in a record feed, including text posts, link posts, and content posts. This object is available in API version 21.0 and later. This object replaces FeedPost.
ConnectApi.FeedElement:
In API version 31.0, the definition of a feed expanded to include new objects that didn’t entirely fit the feed item model. The Chatter feed became a container of feed elements. The abstract class ConnectApi.FeedElement was introduced as a parent class to the existing ConnectApi.FeedItem class. The subset of properties that feed elements share was moved into the ConnectApi.FeedElement class. Because feeds and feed elements are the core of Chatter, understanding them is crucial to developing applications with Connect in Apex.