What is "Equity" and how is it different from common law and statutory law?

Defined

Put in a nutshell, it's when the court concerns itself with fairness. More formally, Black's Legal Dictionary defines it as:

Justice administered according to fairness as contrasted with the strictly formulated rules of the common law. [...] A system of jurisprudence collateral to, and in some cases independent of, "law"; the object of which is to render the administration of justice more complete, by affording relief where the courts of law are incompetent [meaning that they do not have the authority, rather than the colloquial definition of `can not by reason of deficiency'!] to give it...

Examples of equitable principles are "estoppel", "constructive trusts", "unjust enrichment" and "rectification". (Please see my blog posts on rectification here, here, here and here for details on rectification. There will be posts later on estoppel, constructive trusts and unjust enrichment.)

Duhaime's legal dictionary (an excellent source of in-depth definitions and explanations) provides an excellent short history of this area of the law, here. In that summary, Duhaime's quotes the famous English jurist and legal commentator Sir William Blackstone when he points out the danger in becoming too attached to equity as a tool of law:

"Law, without equity, though hard and disagreeable, is much more desirable for the public good than equity without law, which would make every judge a legislator, and introduce most infinite confusion, as there would be almost as many different rules of action laid down in our courts as there are differences of capacity and sentiment in the human mind." (I Blk. Comm. 62)

I must confess myself in agreement with Blackstone. A system of public, known and predictable law which uses equity as a tool to adjust the system to ensure fair results is far better than an all-over-the-map system based on thousands of individual judge's views of what is "fair" or not.

Common law v. statute law v. equity ... and different courts?

Duhaime's has this to say about the differences:

Equity law developed after the common law to offset the rigid interpretations medieval English judges were giving the common law.

For hundreds of years, there were separate courts in England and its dependents: one for common law and one for equity (aka Chancery) and the decisions of the latter, where they conflicted, prevailed.

It is a matter of legal debate whether or not common law and equity are now "fused." It is certainly common to speak of the "common law" to refer to the entire body of English law, including common law and equity.

While it might be a matter of debate whether or not common law and equity are fused it is important to note that the courts which apply them are: Ontario does not separate its courts of common law and equity. The Superior Court of Justice, the Divisional Court, the Court of Appeal and Canada's Supreme Court are all courts which can and do apply statutory law and common law and equity.

Previously done:
Common law.
Statutory law.