Draw a molecule and the notation appears beside it; paste a SMILES and the structure appears instead. Both directions run in the page, so nothing is uploaded and there is no wait.
SMILES writes a molecule as a line of text: atoms as element symbols, bonds as the characters between them, branches in parentheses and rings as matching digits. Everything else here is about learning to read and write that line.
Convert between chemical structures and SMILES in both directions, search a list of structures by substructure or SMARTS, and learn the notation step by step. Everything runs in your browser — no structure you open ever leaves your machine — so the tool needs JavaScript.