For questions about the correctness of a proof or the nature of proofs in general.
A proof is a chain of sentences (or wffs) that is formed according to the rules of a proof calculus. Calculi are specific to a given logic, but all have two properties in common: Firstly, it is algorithmically decidable whether a given chain of sentences constitutes a proof in it; secondly, proofs must be truth-preserving: If the premises are true, then so is the conclusion. This second property makes proofs a special kind of deductive reasoning. The most three common kinds of proof calculi are Hilbert style systems, natural deduction and sequent calculus.
In practice proofs are presented in natural language, yet in principle they are formalisable.