What is a riser card/bus extender and why would I need one?
A riser card is a right angle expansion card that is used to extend a slot for card in computer to make room to plug it in. They are most commonly used in low-profile, 1U and 2U rackmount chassis or embedded systems. Riser cards cards plug into their respective bus (they are available for PCI, PCI-X, AGP, AGP Pro, PCI Express, ISA, or other busses) and rotate the peripheral cards, that are plugged into the riser card, so that they are parallel with the motherboard. Riser cards are available in 1-slot passive risers all the way up to 3-slot passive riser cards for 2U rackmounts.

For users that only have one PCI slot available on their motherboard but require more, active riser cards are available with PCI bridge chips on them to expand the PCI bus.

Bus extenders are also riser cards however they do not always rotate a card so it is parallel with the motherboard. They are used like extension cords for power outlets. They basically extend the bus to another location so it is more convenient for the user.

Users who have low profile applications such as 1U or 2U rackmount systems, embedded systems, or custom projects where their peripheral cards plugged into the onboard expansion slots are too tall for their application. Or if that particular application requires them to relocate their card to a different portion of their enclosure. Riser cards and bus extenders can adapt your expansion slots so that they work for you.
 
What are the different types of riser cards/bus extender?

Straight Extender
Straight-thru bus extenders are used to increase the height of your expansion slot as well as product development and debugging. They are also widely used by production testing of motherboards and I/O boards. Some are utilized as wearout units for the connector or gold fingers in test fixtures.

Right Angle Risers
Right Angle Extenders/Risers are most popular in 1U and 2U rackmount chassis systems or low profile embedded applications that require full height peripheral cards.

Flexible Extenders
Flexible bus extenders can be used to relocate bus expansion slots to another location. Flexible extenders in custom enclosures that have non-standard mounting locations where a standard riser would not work.

Bus Isolation Extender
Bus Isolation Extenders are innovative tools for developing and testing of PC add-on boards. They allow the board under test to be added and removed from the system bus, without having to power down the entire system. They offer A-Side Right Placements time and labor savings during product development and production testing.

Riser Card Directional Orientation
Riser Card Directional Orientation only applies to single and multi-slot right angle riser cards. The orientation of a right angle riser card determines what direction the perihperal card will face, whether is be towards the cpu or away from the cpu in some cases. For users using right angle riser cards, they can either choose from A-Side Right Placement risers or B-Side Left Placement riser cards.

A-Side Right Placement is where the peripheral card plugs into the riser card and is facing the left side (viewed from the front of the motherboard) with the solder side up.

B-Side Left Placement is where the peripheral card plugs into the riser card and is facing the left side (viewed from the front of the motherboard) with the solder side up.

Some riser cards have intercchangeable orientations but with different voltages for each direction. For example the PCITX4-1-B5/A3, after the model number, "B5" means when this riser is plugged into the B-Side Left Placement it can support 5V cards, "A3" means when it is plugged into the A-Side Right Placement it can support 3.3V cards. Both orientations and voltages support universal voltage cards that are keyed for both 5V and 3.3V cards.