The keys distributed by the key server are almost always provided as part of a cryptographically-protected identity certificate containing not only the key but also 'entity' information about the owner of the key. The certificate is usually in a standard format, such as the OpenPGP public key format, the X.509 certificate format, or the PKCS format. Further, the key is almost always a public key for use with an asymmetric key encryption algorithm.
The first web-based PGP keyserver was written for a thesis by Marc Horowitz, while he was studying at MIT. Horowitz's keyserver was called the HKP Keyserver after a web-based protocol it used to allow people to interact with the keyserver. Users were able to upload, download, and search keys either through the -based HKP protocol on port 11371, or through web pages which ran CGI scripts. Before the creation of the HKP Keyserver, keyservers relied on email processing scripts for interaction.
A separate key server, known as the PGP Certificate Server, was developed by PGP, Inc. and was used as the software (through version 2.5.x for the server) for the default key server in PGP through version 8.x (for the client software), keyserver.pgp.com. Network Associates was granted a patent co-authored by Jon Callas (United States Patent 6336186) on the key server concept.
To replace the aging Certificate Server, an LDAP-based key server was redesigned at Network Associates in part by Randy Harmon and Len Sassaman, called PGP Keyserver 7.0. With the release of PGP 6.0, LDAP was the preferred key server interface for Network Associates’ PGP versions. This LDAP and LDAPS key server (which also spoke HKP for backwards compatibility, though the protocol was (arguably correctly) referred to as “HTTP” or “HTTPS”) also formed the basis for the PGP Administration tools for private key servers in corporate settings, along with a schema for Netscape Directory Server. It was later replaced by the new PGP Corporation Global Directory.
The most important universally accessible key servers are those computers, located around the world, which store and provide OpenPGP keys over the Internet for users of that cryptosystem. In this instance, the computers can be, and are, mostly run by individuals as a pro bono service, facilitating the web of trust model PGP uses. There are also multiple proprietary public key infrastructure systems which maintain key servers for their users; only their users are likely to be aware of them at all.
For many individuals, the purpose of using cryptography is to obtain a higher level of privacy in personal interactions and relationships. It has been pointed out that allowing a public key to be uploaded in a key server when using decentralized web of trust based cryptographic systems, like PGP, may reveal a good deal of information that an individual may wish to have kept private. Since PGP relies on signatures on an individual's public key to determine the authenticity of that key, potential relationships can be revealed by analyzing the signers of a given key. In this way, models of entire social networks can be developed.
The OpenPGP keyservers developed in the 1990s suffered from a few problems. Once a public key has been uploaded, it is difficult to remove. Some users stop using their public keys for various reasons, such as when they forget their pass phrase, or if their private key is compromised or lost. In those cases, it was hard to delete a public key from the server, and even if it were deleted, someone else can upload a fresh copy of the same public key to the server. This leads to an accumulation of old fossil public keys that never go away, a form of keyserver "plaque". Another problem is that anyone can upload a bogus public key to the keyserver, bearing the name of a person who in fact does not own that key. The keyserver had no way to check to see if the key was legitimate.
To solve these problems, PGP Corp developed a new generation of key server, called the PGP Global Directory This keyserver sent an email confirmation request to the putative key owner, asking that person to confirm that the key in question is his. If he confirms it, the PGP Global Directory accepts the key. This can be renewed periodically, to prevent the accumulation of keyserver plaque. The result is a higher quality collection of public keys, and each key has been vetted by email with the key's apparent owner.
These are some keyservers that are often used for looking up keys with "gpg --recv-key"