3D: Everybody knows what 3D is, but what is it in a computer? 3D is generally what is used to refer to the capabilities of the video card. Today's video cards use a variety of instructions built into the video card itself (not software) to achieve more realistic graphics in computer games that appear to have depth. Most of today's video cards have these capabilities, but there are incredibly large differences between specific cards. These 3D capabilities are generally used for games, but high-end video cards are used for creating 3D models and 3D animation. Many of these cards cost well over $1,000 and are not intended for home use.
3DNow!: AMD's set of additional instructions that they integrated into their CPUs. Similar to MMX and SSE/KNI, these instructions are intended to speed up CPU performance. These only appear in AMD CPUs.
AGP: Accelerated Graphics Port is a high-speed connection only used by video cards, so there is only one of them in a computer (older computers do not have AGP). It is faster than PCI but slower than PCI-Express and has direct access to system memory so that the computer's memory can be used in addition to the video card's memory. There now is also AGP 4x and 8x. Default AGP speed is 66 MHz.
AMR: Audio Modem Riser and CNR (Communications Network Riser) are slots on motherboards that can accommodate low cost sound cards, network cards, etc. So far there isn't really much available for these slots, and they're not likely to be used much in the future either.
AT/ATX/Extended ATX: These are three standard types of motherboards, cases, and power supplies. An ATX motherboard generally must be used in an ATX case with an ATX power supply. When upgrading your computer, you need to know what type you have and what type you will be getting. If they're not compatible they won't work. ATX is the norm, particularly for Pentium III, IV, and Celeron computers.
BIOS: This is the Basic Input/Output System and is installed on the computer's motherboard. It controls the most basic operations and is responsible for starting your computer up and initializing the hardware. It is data that is usually held in a ROM chip, which can be updated by "flashing". BIOS upgrades may correct errors, support new CPUs, support new hardware, etc.
Burner (CD-ROM): This is a device that allows you to save data to a CD-ROM. Special CD-Rs are required for this. They also allow you to make backup copies of your CDs. There are a large variety of types, including CD-R or CD-R+RW. The latter has support for rewritable CDs that can be erased and rewritten to, while CD-R only drives can only write to their CDs once. Like CD-ROM drives, burners can be IDE or SCSI. SCSI is definitely preferable when it comes to burners, but a SCSI card is required. Burners are generally quite picky and must have a constant stream of data to work properly. If that stream is interrupted, the burn will fail. This is one reason why SCSI burners tend to be better; SCSI devices can deliver a more reliable stream of data than IDE, while other applications are being run as well.
Bus or System Bus: This is just a collection of wires that transmit data from one component to another.
Bus Speed: This is a speed measured in Megahertz that determines how fast the memory and CPU run. The only "official" bus speeds supported by Intel are 66 and 100. However, numerous others exist (75, 83, 103, 112, 124, 133, 153, etc.). High-quality memory is required for the higher bus speeds. The bus speeds usually determine the speed of the PCI and AGP buses as well. The default PCI bus speed is 33 and the default AGP bus speed is 66 MHz. The CPU speed is determined by a combination of the bus speed and multiplier (i.e. 100 bus speed x 4.5 multiplier = 450 MHz CPU speed).