BS EN ISO: British Standard/European Standard/International Organization for Standardization. To put it another way, the British national equivalent of ISO Standards.
They are international standards that have been approved by the British Standards Institution. They can be technical specifications or certification procedures. The term "standard" is used instead of "specification", because a standard describes what something should do, not how it should look. For example, there is a BS EN 12073:2003 standard describing how toilets should be designed and constructed. There is also an ISO 9126 standard that describes how toilets should be maintained.
There are two types of BS standards: primary and secondary. Primary standards are mandatory requirements for all manufacturers or suppliers of goods or services to which they apply. For example, all manufacturers of windows must comply with the requirements of BS 7300:2009, which is a primary standard. Secondary standards are recommended practices that one manufacturer or supplier may choose to adopt or not. For example, some manufacturers may decide to use thicker glass in their windows than the minimum requirement in order to reduce the risk of injury in the event of a crash. These decisions are up to them. However, they cannot ignore the requirement stated in the primary standard.
BS is an abbreviation for British Standard, EN is an abbreviation for European Norm (i.e. European Standard), and ISO is an abbreviation for International Organization for Standardisation (i.e. International Standard). Some have a mix of the acronyms as a prefix, which might cause confusion!
In addition to these standards, some companies use their own internal standards that they claim are equivalent to those standards. For example, a company's standard wire gauge may be called 6 AWG but it may not be the same as the electrical wiring specification defined by BS or EN.
The manufacturer's name should be displayed on any product that states it is approved by one of these organizations. For example, cars labeled "European-approved" have passed tests set by EU countries for safety features such as airbags and child restraints. These cars must comply with EU regulations, which may differ from U.S. regulations.
There are four main types of standards: technical specifications, production requirements, performance criteria, and marketing requirements. Technical specifications describe the function and performance characteristics of a product or service. They can be used by manufacturers to create products that perform the same task more efficiently or consumers to select between similar products. Production requirements specify how much of each product will be made. For example, if one producer makes 100 units of a device, then another producer needs to make at least 101 units in order to meet the demand from customers.
In many circumstances, standards are prefixed with "BS EN," indicating that they are the UK version in English of a European harmonised standard (in some cases, the prefix may be "BS EN ISO" where an international standard has been adopted by Europe as a European standard).
For example, the British Standard for electrical wiring diagrams is called "BS 4761:1983+A1:1985." The word "electrical" before the word "wiring" indicates that this is a standard for electricity-related items such as cables and connectors. The word "British" after the prefix means that this is the UK version of the standard, which differs from the EU version published by ISO.
Other examples of standards with EN as their prefix include BS 7500:2009 Oil and gas production facilities - Code of practice, BS 5850:2010 Specification for steel wire rope, and BS 6978:2011 Designation of marks on glass containers - General requirements.
These standards can be found on the Business Standards website. They can also be obtained from any of the listed stockists in the UK.
If you are looking up a standard that does not have EN as its prefix, it may still be useful to read though it as some standards are only relevant to specific industries or products.