2D bulbs (also referred to as DD bulbs) offer the same brightness as fluorescent and LED tube lights, but they are turned into more compact shapes, making them more convenient and easier to install.
Like standard straight tubes, 2D bulbs are available in fluorescent and LED versions. LED 2D bulbs are usually made to retrofit to fluorescent fittings, meaning that you don't need to replace the entire light fixture when upgrading your lighting.
2D lamps are available with two different cap types: the GR8 or the GR10q. The easiest way to tell which type of cap your bulb has is to count the pins on its underside: if there are 2 pins, it is a GR8 connector, and if there are 4 pins, it is a GR10q.