Public Key Infrastructure
Acronym: PKI
The Public Key Infrastructure (PKI) is the software management system and database system that allows to sign certifcate, keep a list of revoked certificates, distribute public key,... You can usually access it via a website and/or ldap server. There will be also some people checking that you are who you are... For securing individual applications, you can use any well known commercial PKI as their root CA certificate is most likely to be inside your browser/application. The problem is for securing e-mail, either you get a generic type certificate for your e-mail or you must pay about USD100 a year per certificate/e-mail address. There is also no way to find someone's public key if you have never received a prior e-mail with his certificate (including his public key).