Going-to future is a term used to describe an English sentence structure referring to the future, making use of the verb phrase to be going to. Most other languages use the same sort of structure to form a future tense.
Origin
The going to future originated by the extension of the spatial sense of the verb 'go' to a temporal sense (a common change - the same phenomenon can be seen in the preposition before). The original construction involved physical movement with an intention, such as I am going [outside] to harvest the crop. The location later became unnecessary, and the expression was reinterpreted to represent a near future.
The colloquial form gonna is a relaxed pronunciation of going to. For example, "This is gonna be awesome!". This now forms a clear separation of the locative and temporal senses of the expression: while "I am gonna swim" is syntactically similar, a sentence like "I am gonna the beach" is not.
Formation
subject + be (in the proper form for the subject) + going to + verb + any other information
Usage
Going-to future is used when the speaker wishes to express certainty about the future based on evidence or fact from the present or the speaker's opinion: "If you do not stop, you are going to be caught by the police and hauled back to jail." "Our houses are going to be swept away by the impending storm." (Here, will can be substituted for going to without changing the sentences' meaning.)