Help, aid, assist and their corresponding nouns help, aid, assistance are often used with little distinction as meaning (for the verbs) to furnish another person or thing with what is needed (as for the accomplishment of work or the attainment of an end) or (for the nouns) the support so furnished. All usually imply cooperation or a combination of effort.
Help, however, carries a stronger implication of advance toward the end or objective than do the others.
Aid strongly suggests the need of help or relief and therefore sometimes imputes weakness to the one aided and strength to the one aiding.
Assist, which seldom loses its original implication of standing by, distinctively suggests a secondary role in the assistant or a subordinate character in the assistance; thus, a deputy assists rather than aids his superior; a good light assists the eyes in reading.