a decider
A decider is a Turing machine that always finishes. Hand it any input and it is guaranteed to halt, with a definite yes (accept) or no (reject), in finite time. It never gets stuck in an infinite loop. If a general Turing machine is a worker who might or might not ever put down their tools, a decider is one who always, eventually, downs tools and hands you a verdict.
Formally, a decider is a Turing machine M that halts on every input string: for each input it enters q_accept or q_reject after finitely many steps, never looping. The language a decider recognizes is automatically decidable, because the always-halting guarantee plus correct accept/reject behavior is exactly the definition of deciding. So 'decider' is the machine, and 'decidable' (or recursive) is the property of the language it handles. Designing a decider often means adding bookkeeping that forces progress, for example crossing off symbols so the machine cannot revisit the same situation endlessly.
The decider is the honest workhorse of computability: an algorithm in the everyday sense, something that always terminates with the right answer. The whole drama of undecidability is about problems for which NO decider can exist, the halting problem being the headline case. A subtlety worth flagging: every decider is a recognizer, but most recognizers are not deciders, because a recognizer is permitted to loop on non-members. Turning a recognizer into a decider is exactly the step you cannot always take, and when you cannot, the problem is undecidable.
A machine that, given a binary string, scans once to its end checking each symbol is 0 or 1 then halts-accepts is a decider: it stops on every input. A universal machine that simulates an arbitrary M on w is a recognizer but NOT a decider, since it loops whenever M loops.
Decider = a Turing machine guaranteed to halt on every input.
Every decider is a recognizer, but not every recognizer is a decider. The extra promise a decider makes is to halt on all inputs, not merely to accept the members; that promise is exactly what undecidable problems make impossible.