An Option is when a producer pays for the right to purchase a screenplay in a set period of time, essentially taking it off the market. In that period (e.g., 6 months, 1 year) they have the exclusive right to shop the script around, hoping to get it sold, financed, or produced. Most options to new screenwriters are for small sums of money; in most cases, a new writer should consider the track record of the producer (thus the likelihood of their script being sold and produced) as being more important than the price.
A sale is an outright purchase of a screenplay, but many sales listed in the trades contain the “guarantee/upfront” amount and the production bonus (e.g. $100,000 against $300,000). One sum is paid up front (usually in steps) and the remainder of the money is paid if and when the film either goes into active development or begins production. This is to insure the studio against overpaying for scripts that will just sit on their shelves, since they buy more material than they can actually make into films.