OLED stands for ‘organic light emitting diode’. It’s a more advanced display tech than regular LCDs with wider viewing angles, faster response, better brightness and reduced power consumption. The OLED electronic viewfinder is a selling point in the Fujifilm X-T1, for example.
While most cameras now let you compose your shots using an LCD screen on the back, there are lots of times when a regular viewfinder is still preferable. For many people it's more natural to put your eye to a viewfinder eyepiece than it is to hold the camera at arm's length. Viewfinders are also more useful in bright light, when glare often makes it hard to see what's being displayed on an LCD screen. Viewfinders come in different types. Some older cameras have 'direct vision' viewfinders, digital SLRs have optical through-the-lens viewfinders while mirrorless cameras use electronic viewfinders.
A tiny blind in the viewfinder eyepiece that stops light entering and upsetting the exposure (normally the eyepiece is covered by your eye). It can be useful for long exposures or other shots where you’ve stepped away from the camera. Some cameras come with a small viewfinder cap fixed to the shoulder strap.
Essentially, this is a tiny LCD display seen through a magnifying eyepiece. They’re used on some bridge cameras and high-end compact cameras, and on many mirrorless cameras. They replace the optical viewing system you get with a DSLR.
A small knob or lever next to the viewfinder which you use to adjust the focus of the eyepiece to match your own vision. The information in the viewfinder should appear sharp without you having to strain to bring it into focus.