What is common for all web services is that they are the machine-readable equivelant to the webpages the site otherwise offers. This means that others who wish to use the data can send a request to get certain data back that is easy to parse and use. Some sites may require you to provide a username/password in the request, for sensitive data, while other sites allow anyone to extract whatever data they might need.
Sorry Jezelll, had to throw a downvote. This is not what I would consider 'plain english' :( I don't have a problem with the intent of your definition, but using words like 'operation', 'remote server', 'invoked' and 'specification' makes it hard for beginners to understand. See Mitchel Sellers answer as an example of 'plain english'. – Dhaust Jun 30 '09 at 23:53
Name servers. Most registrars provide two or more name servers as part of the registration service. However, a registrant may specify its own authoritative name servers to host a domain's resource records. The registrar's policies govern the number of servers and the type of server information required. Some providers require a hostname and the corresponding IP address or just the hostname, which must be resolvable either in the new domain, or exist elsewhere. Based on traditional requirements (RFC 1034), typically a minimum of two servers is required.
Typically image hosting websites provide an upload interface; a form in which the uploader specifies the location of an image file on their local computer file system. After pressing a "Submit" button the file is uploaded to the image host’s server. Some image hosts allow the uploader to specify multiple files at once using this form, or the ability to upload one ZIP archive containing multiple images. Additionally, some hosts allow FTP access, where single or multiple files can be uploaded in one session using FTP software or an FTP-capable browser.