Second Life (SL) is a virtual world developed by Linden Lab that launched on June 23, 2003 and is accessible via the Internet. A free client program called the Second Life Viewer enables its users, called Residents, to interact with each other through avatars. Residents can explore, meet other residents, socialize, participate in individual and group activities, and create and trade virtual property and services with one another, or travel throughout the world, which residents refer to as the grid. Second Life caters to users aged eighteen and over, while its sister site Teen Second Life is for younger users.

Built into the software is a three dimensional modeling tool based around simple geometric shapes that allows a resident to build virtual objects. This can be used in combination with the Linden Scripting Language which can be used to add functionality to objects. More complex three dimensional Sculpted prims (colloquially known as sculpties), textures for clothing or other objects, and animations and gestures can be created using external software. The Second Life Terms of Service ensure that users retain copyright for any content they create, and the server and client provide simple digital rights management functions.

There is no charge to create a Second Life account or for making use of the world for any period of time. Linden Lab reserves the right to charge for the creation of large numbers of multiple accounts for a single person but at present does not do so. A Premium membership (US$9.95 per month) facilitates access to an increased level of technical support, and provides a stipend of L$300/week. Avatars may take any form the user chooses, animal, vegetable or mineral, including being made to resemble the person whom they represent. A single Resident account may have only one avatar at a time, although the appearance of this avatar can change between as many different forms as the Resident wishes. A single person may also have multiple accounts, and thus appear to be multiple Residents (a person`s multiple accounts are referred to as alts).

In the past, large portions of the Second Life economy comprised businesses that are now regulated or banned. Changes to Second Life`s Terms of Service in this regard have largely had the purpose of bringing activity within Second Life into compliance with various international laws, even though the person running the business may be in full compliance with the law in his own country. Typically, Linden Lab offer no compensation for businesses that are damaged or destroyed by these rule changes, which can render significant expenditure or effort worthless.

Linden Lab, for a long period, offered OpenSpace regions to users: regions which were purchased in packs of four, with all four running on a single CPU core, intended to be placed next to an existing region to create the effect of larger size. The fee for 4 OpenSpaces was identical to that for a single private region. However, in March 2008, this rule was modified to permit OpenSpaces to be bought individually and placed elsewhere, as well as increasing the prim load each one could handle. OpenSpaces were made available for a US$415 downpayment plus a US$75 monthly fee. In October 2008 Linden Lab announced that the OpenSpaces being used for this purpose were being misused; there was in fact no technical throttle limiting their usage. Linden Lab raised the monthly fee per OpenSpace to US$125, the same cost as half a region; added an avatar limit of 20; and renamed it to Homestead. A week after the initial announcement Linden Labs stated its intention to add technical limits. A revised Openspace product, with far fewer prims, a no-residency rule, and costing the same monthly amount, was announced In May 2009, Linden Lab announced they were "grandfathering" OpenSpace sims.