The Cambridge Grammar of the English Language (Huddleston, Pullum et al, 2002) describe three broad categories of ɪʟʟᴏᴄᴜᴛɪᴏɴᴀʀʏ ғᴏʀᴄᴇ with which a clause or sentence may be used:
These are meanings derived from the pragmatics of the utterance of a clause within a given context. These categories of illocutionary force are neither primary, exclusive nor exhaustive. There are utterances which convey other types of illocutionary force, such as promises, for example. And the three broad categories of illocutionary force may also be further subdivided into more finely grained categories. For example, a directive: Chop the parsley may be uttered with the force of an instruction, a command, a request, advice and so forth.
Closely related to the notion of illocutionary force is the syntactic system of ᴄʟᴀᴜsᴇ ᴛʏᴘᴇs. These are syntactic categories of clause, which are broadly associated with different types of illocutionary force. The five major clause types in English are:
- declarative (associated with statements)
- closed interrogative (associated with closed questions)
- open interrogative (associated with open questions)
- exclamative (associated with exclamative statements)
- imperative (associated with directives)
As we can see, imperatives are associated with directives. However, directives may also take the form of declarative, exclamative or interrogative clauses.
In terms of illocutionary force, a request is normally analysed as being a type of directive. When the Original Poster asks what type of sentence a request is, the answer is that in terms of illocutionary force, it is a type of directive. In terms of clause type it could be a declarative, closed or open interrogative, exlcamative or imperative clause. Notice that these pragmatic and syntactic categories are best applied to clauses, not sentences, because a single sentence may have different clauses belonging to different pragmatic and syntactic categories:
- It would be great if you lent a hand, or are you busy right now?
The sentence in the utterance above consists of a declarative directive and a closed interrogative question, for example.
It is probably a good idea to consult a vetted grammar source instead of Wikipedia—especially a Wikipedia entry that takes a definition of sentence from Dictionary.Com as its opening description, organising principle and source of linguistic categorization.