In maritime law, the expression general (or gross) average is used in relation to certain acts, to the losses they cause, and to the rights of contribution to which they give rise. A general-average act consists of any sacrifice or expenditure made intentionally and reasonably to preserve property involved in a sea voyage. For example, the jettisoning of some of a ship's cargo to keep it afloat during a storm is a general-average act. The loss directly resulting from a general-average act is called general-average loss and is borne proportionately by all whose property has been saved. The owner of jettisoned cargo, for example, is entitled to a contribution from other cargo owners as well as the shipowners; such a contribution is called a general-average contribution. The principle of general average is common to the laws of all maritime nations, but the detailed rules are not uniform. To overcome conflict of law, a standard set of rules was agreed in the 19th century at international conferences of shipowners and others held at York and Antwerp. These, known as the York–Antwerp rules, do not have the force of law, but it is common practice to incorporate them (as subsequently amended) in contracts of affreightment, thereby displacing national laws. The basic principle is that an insured who has suffered a general-average loss may recover the whole of it from his underwriters without enforcing his rights to contribution; these become enforceable by the underwriters instead.
By contrast, particular average (also called simple or petty average) relates purely to marine insurance. It consists of any partial loss that is not a general-average loss (for example, the damage of cargo by seawater). It is therefore borne purely by the person suffering it and is frequently covered by a policy only in limited circumstances.
A ship sold free from average is free from any claims whatsoever.
http://www.jus.uio.no/lm/cmi.york.antwerp.rules.1994/doc.html The latest version of the York–Antwerp Rules, as prepared by the Comité Maritime International