The document discusses key concepts in contracts for the sale of goods under the Sale of Goods Act 1930 including: 1) A contract of sale involves the transfer of property in goods from a seller to a buyer in exchange for a price. 2) A sale involves the immediate transfer of property, while an agreement to sell involves transfer at a future time or when conditions are met. 3) Essential elements of a valid sale contract are two parties, goods, price, and transfer of property. 4) Key differences between a sale and hire purchase agreement are that a sale transfers full ownership to the buyer, while a hire purchase agreement transfers ownership only after installments are paid in full.