A failure to do what a binding agreement required - and the question a court actually asks is not whether someone broke a promise but what the loss was and whether it could have been avoided.
A breach of contract is a failure, without a legal excuse, to perform what an enforceable agreement required. Before a court reaches the breach it has to find a contract at all: an offer, an acceptance, something of value exchanged on both sides, and enough definite terms to know what was promised. A great deal of business litigation is lost at that first step, because the parties were still negotiating, or one side's "agreement" was an email that the other side never accepted.
Not every breach is equal. A material breach goes to the heart of the bargain and generally lets the other side stop performing and sue; a minor breach entitles them to compensation but not to walk away. Which side of that line a failure falls on is usually the real dispute.
The remedy is ordinarily money measured by the loss the breach caused - the position the injured party would have been in had the contract been kept - and it comes with a duty on the injured party to take reasonable steps to limit that loss. A party who could have found a replacement supplier and did not will generally not recover the loss that a replacement would have avoided. Courts order a party to actually perform, rather than pay, only in unusual cases such as the sale of a unique piece of land.
Two moments justify advice: when a counterparty stops performing and you are deciding whether to stop too, because treating a minor breach as grounds to walk away can make you the party in breach; and before signing anything with a limitation-of-liability, liquidated-damages or choice-of-law clause, since those three clauses decide most of what a breach will ever be worth.
Choose your state. Each link opens the directory page for the city in that state with the most currently published law firms in this practice area; a +n beside the city is how many other cities in the state also have one. The list is generated when this page loads, so a state whose listings have lapsed drops out rather than becoming a dead link.