Different names, same job. A decoder turns codes (hex, binary, decimal) back into readable text; an encoder does the reverse; a translator or calculator is just either of those with a friendlier name. The tool above does all of it — pick a source and target format and type.
To look up a single character instead of converting whole strings, the ASCII keyboard map shows every key's code interactively — and if you're deciding between encodings, start with the ASCII vs Unicode comparison.