Hello Pigfoot
'A clause' is any part of speech that retains a form similar to a sentence. In traditional grammar, only "finite clauses" are regarded as clauses. But may grammarians nowadays classify '-ing' constructs and infinitives as "infinite clauses". "A finite clause" means that the form of the verbal used in the clause changes depending the single/plural of the subject and the tense. Infinitives and '-ing' don't change in the form whatever the tense and the subject's number may be. So they are called "infinite clauses".
(EX) He worked so hard [that he saved much money]. A finite clause, because 'saved' is a finite verbal.
(EX) He worked so hard [to save much money]. An infinite clause, because 'save' is an infinite verbal.
(EX) [Working so hard], he saved much money. An infinite clause, because 'working' is an infinite verbal.
paco