Broadly speaking, “parables” encompass a variety of types of discourse which are used to give insight into the Kingdom of God that was inaugurated with the work of Christ. Some of these types that are broadly considered “parables” are similes (Matt 10:16) and metaphors (Matt 5:13; 12:34; John 14:9). What are quite often read as “parables” such as Mark 4:26-29 and 30-32 can be more accurately called “similitudes” because they are extended similes or metaphors with no character or story plot.

A helpful way to restrict the term is to limit “parable” (Greek: parabole) to “story-parables.” These are recognizable stories with characters and plots used to expressly teach hearers/readers about the Kingdom of God, its King, and its citizens. Popular examples of true parables are those of the “Prodigal Son” and “The Good Samaritan.” These “story-parables” are usually more difficult to understand and exegete (interpret).

(Adapted by Mark Vander Pol from lectures given by S. M. Baugh)

Source: http://www.whitehorseinn.org/terms-to-learn-table-of-contents.html