A ‘clause is a group of words that has a subject and a predicate (verb). A clause can function as a sentence by itself or as part of a sentence.
There are many types of clauses, depending on what the purpose is. There are ‘main clauses’ which can stand as sentences on their own, and there are ‘subordinate clauses which add information to the main clause but cannot stand on their own.
There are clauses whose type is determined by the role which they serve within the sentence, for instance there can be an ‘adjective clause’ which will modify the noun, or an ‘adverb clause’ which serves to modify the verb in a sentence. Similarly a ‘noun clause’ will complement the noun of that sentence.
There are four basic kinds of sentences, Declarative, Imperative, Interrogative, and Explanatory. There are simple and compound sentences as well and Complex and Compound-Complex sentences.
As well as well constructed and composed sentences there are some sentences that have to be classified as errors and these are those sentences which are not constructed according to English syntax rules. These are called Sentence Fragments, and Run-on sentences, and they can be recognized by their not masking clear sense in the context.
See the related articles for more details.