Answer:
For a contract to be legally binding it must contain four essential elements:
an offer
an acceptance
an intention to create a legal relationship
a consideration (usually money).
However it may still be considered invalid if it:
entices someone to commit a crime, or is illegal
is entered into by someone that lacks capacity, such as a minor or bankrupt
was agreed through misleading or deceptive conduct, duress, unconscionable conduct or undue influence.