An app interacts with a Solana cluster by sending it transactions with one or more instructions. The Solana runtime passes those instructions to programs deployed by app developers beforehand. An instruction might, for example, tell a program to transfer lamports from one account to another or create an interactive contract that governs how lamports are transferred. Instructions are executed sequentially and atomically for each transaction. If any instruction is invalid, all account changes in the transaction are discarded.
To start developing immediately you can build, deploy, and run one of the examples.