I want to connect device X but it doesn't fit.
Keyboard - Keyboard ports come in two sizes. The old style, and therefore larger is called AT. The newer, and of course smaller port, is called PS/2. They are compatible with each other and an adapter can be used to switch between the two. New mice also work in a PS/2 port. On desktop computers there may be 1 AT port and 1 PS/2 port, the first is for the keyboard, the second for the mouse. On systems with 2 PS/2 ports as in the picture, the top is for the mouse and bottom for the keyboard as the icons suggest. Green connectors being for the mouse and purple being for the keyboard.
Monitor - This is the connector that the monitor cable is plugged into. Some systems may have two of these. If nothing appears on the monitor plug the cable into the other connector. The connector may also be referred to as HD-15 or the video port.
Serial and Parallel ports - Serial ports come in two different sizes, 9 pin and 25 pin, called DB-9 and DB-25, respectively. Serial ports, sometimes referred to as COM ports, on the back of a computer are Male connectors, and are primarily the only male connectors on the back of a computer. This is because serial connections are quite old (technologically speaking) and others did away with this idea since having pins sticking out the back of things is just asking for them to get broken off. Serial connections are used for a majority of devices except printers. Although some printers do have serial capabilities. For more on the differences between Serial and Parallel check out the geek section
Parallel ports are 25 pin only and are Female. These connectors were originally designed to be used only with printers. Since then manufacturers have made other devices work on this connector, such as scanners, zip drives and video capture devices.
Although you can go down to your local computer store and pick up adapters and/or gender changes to make your cable fit into the back of your computer. You need to make sure not to cross between serial and parallel connections. If you do, it won't work. Another thing to keep in mind about cables is what is called a 'Null Modem' or 'File Transfer' cable. These cables have the wires inside intentionally crossed is such a manner so the send and receive wires, swap places on the other end. These are used for connecting two computers together to transfer files or other information, although significantly faster ways are available. If you need help with the male vs. female designations, think 'birds and bees'. If the doesn't help, ask your parents, this site is about computers, not biology.
RJ-11 - Also known as the Phone Jack. This is just the name of the connector for the phone line. If your computer has a modem in it this is where you would put the phone line. Most systems have two jacks side-by-side. One is usually labeled 'Phone', this is where you can daisy chain another phone or fax machine on the same line. The second connection is sometimes labeled 'Line' or 'Tel Co', and is the connector that the phone line from the wall should be connected to.
USB - Universal Serial Bus ports are the big thing right now. Almost every computer within the last few years has them. Just about any device can be adapted to work with USB. It is significantly faster than Serial or Parallel ports and can host multiple devices per port. All that is need is a hub to give more connections. It has a through-put of 12Mbps vs. Serial's maximum of 115Kbps, over 10 times faster. USB is the epitome of Plug and Play, with no settings to configure to get devices working.
Firewire - or IEEE1394 or i.Link as Sony likes to call it. These ports are being found and more computers. Firewire is similar to USB in many aspects. It was designed for high speed digital video transfers but is great for any type of data transfer. It is primarily used for external Hard Disk drives and digital video cameras. Unlike USB, it does not need a central device to control the connetion (i.e. a computer) so it is able to link 2 computers together via a standard cable without any extra device. There are two types of connections for Firewire, a 4 pin and a 6 pin varieties. The 6 pin connection is commonly found on desktop systems and larger devices. The send and recieve signal wires are twisted pairs and use 2 wires each, that plus 2 wires for power gives us our six wires in the connector. The 4 pin connectors are usually found on self powered devices, smaller devices, and notebook systems. To reduce the size and pin count of the connector the 2 wires for power are removed.
Serial vs. Parallel - Serial and Parallel refer to how information is passed from one device to another, typically through a cable. Serial transfers only need two wires. One for sending and a second for receiving. Grounding is typically through the case of each individual case or any "extra" wires including in the cabling. To transfer 1 byte a serial connection transfers 1 byte plus 1 stop bit which comes out to 9 bits (1 byte = 8 bits). Each bit is either a 1 or a 0. Since we can easily change the voltage on a wire, each bit will be represented by either 'high' (1) or 'low' (0) line voltage. So, to transfer our single byte of information a serial connection must send 9 bits down 1 line, one at a time. We measure the time this takes in cycles, and one thing happens each cycle. So, for serial it takes 9 cycles to send 1 byte of information. Which is approx. 0.000184 seconds. with a 56k modem. For parallel, as the name implies, things are done in parallel (at the same time). Parallel tends to have a specific number of sending wires, which may or may not also be used for receiving. For our case, we are using 8 sending wires, 1 for each bit in the byte. So, for each byte that we want to send we only need 1 cycle. Which means in the time it takes to send 1 byte with serial connection we can send 9 bytes through a parallel connection.