With a secure web server, clients can connect to your server secure in the knowledge both that it is who it claims to be and that the transaction is well-encrypted so their data is safe. The best way of doing this is with Apache 2, the leading Linux web server software, and Secure Sockets Layer, a secure communication protocol. Transport Layer Security (TLS) is the successor to SSL, but they work in basically the same way. I’ll refer from here on just to SSL.
SSL is a protocol for cryptographically securing transactions between a web browser and a web server. In most cases, only the server end is authenticated, which means that the client has a guarantee that the server is who it claims to be, but not vice versa. However, once the connection is established, both ends are secure, as only the client and the server have access to the key material. This makes sense since for many transactions, the server doesn’t care who the client is, as long as it stays the same client throughout the transaction. If you do care about client authentication, it is possible to use client SSL certificates (or htaccess, Kerberos, or other similar methods), but that won’t be covered in this article.
As a client, obviously you do care that you’re sending whatever private data you wish to encrypt to the person (server) you think you’re sending it to. Hence the server, not the client, being authenticated. You also care about preventing a third party from accessing your data as you send it. SSL provides both of these types of security.