A verbal agreement may constitute a binding agreement or
contract although it must meet certain requirements. Whether the
parties have a valid contract is a determination that must be made
by a judge. Some agreements must be in writing in order to be
binding. These include transactions regarding real estate and an
agreement to make a Will.
Depends on what you're agreeing.
Agreements for the sale of land, for example, must be in writing. The same is true for contracts of guarantee.
If you're agreeing "i'll lend you $x if you pay it back at y%/annum" then your only difficulty will be in proving that you both did actually say that, and avoiding a "what? I don't remember agreeing to that" situation.