Electronic commerce, in short e-commerce, consists of the buying and selling of products or services over electronic systems such as the Internet and other computer networks (Source: Wikipedia).

The basic requirements to sell over the Internet are principally the same as in an off-line business:
Registration: Your business needs to have a name, a seat  (real address) and must be registered (according to national rules, depending where the seat is). Upon registration, you receive a VAT number, your unique business identity.
Observation of common business rules: Law on unfair commercial practices, know-how and trade secrets, accounting etc.

Requirements specific to e-Commerce:
A difference is made between B2B = business to business, or business among "equals", as opposed to B2C = business to consumer, where the consumer is considered the weaker part and, therefore, enjoys special protection.

Rules applicable to both B2B and B2C:
For a general description, see e-commerce
Commercial website: How to set up a commercial website? - Domain name - Website privacy laws
VAT: VAT rules for on-line commerce

B2C Consumer protection rules:
On-line commerce applies the rules for distance selling (like buying from a catalogue; without the presence of the seller and buyer at the same time). These rules can mean considerable hardships to small and micro enterprises (e.g. not knowing when a contract is final!). Thus, a quick look to these rules can save long troubles!

For consumer rights on-line, consult eYOU Guide, DG Information Society's portal for legal questions with regard to on-line activities!

B2C Requirements:
Need to provide full information on the website. If this is not observed, the consumer has a longer period to cancel the contract (3 months instead of the ones laid down in the cooling off periods. For details, see consumer protection.
Cooling-off periods: periods during which the consumer can cancel the contract without giving any reason
Applicable jurisdiction is that of the consumer (where the consumer lives) and not the seat of the commerce (as in B2B if not otherwise negotiated)
Commercial communication / data protection / privacy: These are treated in detail under spam
e-Contracts: see details under e-contracts.
Trust & Security: European Parliament report on e-confidence and e-commerce