Online play, also known as online multiplayer, refers to a mode in which multiple players can play a game together or against each other over an Internet connection.
Online matches are usually hosted on official dedicated game servers which are accessed from a central master server, but some developers also allow independent community servers to be run and broadcast worldwide.
Most often, access to official servers requires a valid account tied to a store profile, email address, or a serial key. Accounts can be banned by the server operators due to malicious activity, revoking multiplayer access.
The latency (ping) determines how long the signal takes from the computer to the server and back. Actions in game arrive faster at the server if the latency is low. It can be very frustrating to play with a high latency, because the game becomes very unresponsive. Most players consider pings below 50 ms as acceptable, games usually become uncomfortable with a latency above 100 ms.
Latency can depend on connection quality, server issues and geographical distance between a player and the server. Some games have their multiplayer servers split to regions to minimize this effect, but the side-effect is a segmentation of the playerbase.
Official master servers are often shut down due to low activity or internal issues at the company hosting the servers, regardless if they are run in-house or through a third party like GameSpy, MPlayer.com or Quazal.
In many cases, unofficial alternatives are set up by fans; see the Network section in individual articles to see if this is the case for a given game. Another alternative is using LAN play if the game supports it.