Apache
Free and open source web server software - and probably the most commonly used web server software. The web server software is what allows the web server to accept and respond to requests for web pages from users.

Backups
A backup system automatically makes copies of the files that make up a website as well as other important files, with the goal of allowing a quick recovery in case of a loss of data whether through a security problem or some sort of hardware/software failure. There's a lot of variance in what sort of backups that web hosts offer - from none at all to nightly or even more regular backups. Generally there's a cycle that allows for a range of files from different dates to be available.

Bandwidth
"Bandwidth" is typically used to refer to the amount of data transfer allowed in one month under a web hosting plan. This is usually measured in gigabytes. Whenever a visitor accesses your website, at least one (and usually lots more) files will be transferred from the web server to the visitor's computer. Generally the web host has a limit on the total allowed size of the files transferred - and excess usage is charged at some predefined rate.

Control Panel
A "control panel" is a web-based system that allows a user to more easily control their web hosting account, allowing things such as setting up email accounts, adding databases, changing apache settings, etc. There's a wide variety of control panel software, and often the functionality will vary based on what the web host allows their customers to do with their site.

Colocation
Colocation or "equipment colocation" allows a company or organization to place their equipment in a web hosts data center, essentially renting space, power, bandwidth etc - but not renting an actual server. This allows the company to use their own custom equipment, but take advantage of the benefits of the web host's data center (reliability, security, 24/7 staffing, etc).

Dedicated Hosting
When a single client has their own web server dedicated to their sole use, it is dedicated hosting.

Dedicated Server
A web server that is dedicated to the use of a single customer.

Disk Space
This is the amount of space allocated to the customer's use under a web hosting plan. Typically measured in megabytes or gigabytes (one gigabyte is about one thousand megabytes).

DNS
"Domain Name Service" - this is the network of servers that allow the user's browser to translate a domain name into an IP address which it can use to contact the web server that hosts the site at that domain. Some hosts allow users control over their own DNS servers, which allows for a greater level of control over their domains and subdomains.

Domain Name
A domain name is actually an alias to an IP address. Every web server will have at least one IP address that visitors use to visit the website(s) hosted on that machine. The domain name acts as a more easily remembered "name" for that IP address, and the translation from domain name to IP address is handled behind the scenes.

FTP
Stands for "File Transfer Protocol" - a common way to exchange files between two computers over the Internet. It is probably the most commonly used method to upload or download files from a web server.

IMAP
IMAP is an email protocol that allows users to receive using an IMAP email client. The biggest difference between POP and IMAP is that with IMAP all email, folders, etc are stored on the server, rather than on the client's local computer.

IP Address
A numeric address on the Internet. Every web server will have at least one IP address. In some ways they're analogous to a phone number - to call another phone, you need it's phone number.

LAMP
Stands for commonly used set of web software: Linux (operating system), Apache (web server software), MySQL (database software), PHP (scripting language). Variants include MAMP (Macintosh operating system) and WAMP (Windows operating system).

MySQL
MySQL is a popular free and open source database application, commonly used on linux-based web servers. It allows web applications to store and manipulate data, and is used by a very wide range of free web-based applications such as WordPress and Joomla.

PHP
PHP is a very commonly used programming language used to build websites. It is arguable the most popular such language used today, and has a huge amount of free software available for it.

POP
POP is an email protocol that allows users to receive email using a POP mail client such as Outlook.

Reseller
A reseller is someone who purchases a hosting plan with the intention of "splitting" it into smaller web hosting plans and reselling them. Web hosts often create special plans for these kinds of customers, giving the plans tools to help manage multiple sub-accounts, more disk space, etc.

Shared Hosting
"Shared Hosting" is one of the more common forms of web hosting, in which many customers "share" the resources of a single server - and also sharing the cost of the server, making it more affordable than other kinds of web hosting. Generally if a web host doesn't specify what kind of hosting they're offering, it's probably shared hosting.

SSH
Stands for "Secure Shell" - a network protocol that allows a secure console-based connection to a remote machine. SSH allows a user to login to a machine and use the command-line to execute commands. It is typically used for web development and server administration.

SSL
"Secure Socket Layer" - a protocol that allows end-to-end encryption of traffic between a user's browser and a web server. Critical if a website plans to do anything that involves information that should be kept private, such as ecommerce.

VPS (Virtual Private Server)
Special software can be used to allow a single server to host multiple "virtual" servers. Each virtual server is totally independent of the others, and is essentially a dedicated server. This provides more reliability, isolation, and flexibility than a standard shared hosting account, but is still usually cheaper than a "real" dedicated server.

Web Server
A web server is a computer that is set up to accept and respond to request for web pages that remote users make using their web browsers.