1. Both are equitable remedies, thus discretionary.

2. Specific performance is a decree of the court ordering a defendant to perform his obligations under a contract.
a) Where damages provide an adequate remedy, specific performance willl not be decreed. (Equity follows the law.)
b) Specific performance will not be granted in the absence of mutuality. (Equity will not assist a volunteer.)
Likewise, there will be no specific performance on behalf of a minor because the contract cannot be enforced against him.
c) There can be no specific performance of a contract for personal services (Trade Union and Labour Relations (Consolidation) Act 1992).
d) The courts have discretion to award damages in lieu of or in addition to specific performance if it 'thinks fit' (Chancery Amendment (Lord Cairn's) Act 1858.

3. In contract law, an injunction is an order commanding a defendant to refrain from breaking his contract.
a) It may be used as an indirect means of enforcing specific performance where, for some reason (i.e. contract of duration - spec. perf. would require constant supervision), the latter is unavailable. For example, where A contracts to only obtain supplies from B, he can be ordered not to obtain them elsewhere.
b) In the case of contracts for personal services injunctions will only be granted in respect of express negative covenants: Lumley v. Wagner 1852 - a signer should not be tied to someone else or 'starve' - injunctions are not used for such purpose.
c) Lord Cairn's Act (above) applies here, too.