Its the Aperture that is denoted by f and it is also known as f stop.

The diffrence between both  is that f/1.4 lets you get more light to your image.

rather f/1.8 lets you get less light in your image.

More the aperture less the light and less the aperture more the light .

and one of the main use of it is the depth of the field.

Suppose you want your background to be blurred at a good rate then use as low apeature as you can.

Hope you will find it helpful.

Types of equipment that are needed to shoot long exposures

A tripod – this is a good piece of equipment to have in most instance, but is a critical piece of equipment for shooting long exposures. Make sure your camera is properly mounted on the tripod before you start shooting.

A camera – obviously you will need a camera, but many people assume you can only do this type of photography with an SLR. Some advanced point-and-shoot cameras can also do long exposures if they have a Shutter Priority function. Take a look in your camera’s manual to see if it has this function, you may be surprised.

Cable release – your camera will be on a tripod, so it should be very still, however, sometimes the action of pressing the shutter release button can cause the camera to move slightly and this movement can cause your image to blur very slightly. You may not notice this on the LCD screen, but when you open the image on your computer, it will be evident. I recommend getting a cable release (also called a remote trigger). It is simply a cable that attaches to your camera and acts as a shutter release button. Using a cable release means you can set up your camera, step away from the tripod and press the button without touching the camera. Cable releases can be wireless too. If you don’t have a cable release or don’t want to buy one, you could use your camera’s self-timer function to trigger the shutter.

Camera settings should be used

Shutter speed – depending on the light in your scene, your shutter time will need to be at least 10 to 15 seconds, or longer if necessary. If you are doing a seascape and the water is moving quickly, then a few seconds may be long enough to make the water look misty.

Aperture – you will want to have your aperture set at anywhere between f/8 and f/16. This again will be determined by how much light is in the scene and how long you want to expose for.

ISO – keep your ISO settings as low as possible, ISO 100 is what I use for long exposures.

It’s normally a good idea to shoot long exposures as the sun is setting, or just after sunset. My suggestion is to be on the scene an hour before sunset. That way you can test some shots, make sure your composition is good and be sure all your settings are correct. 

1. Shooting with 50mm lens can let you shoot at aperture as wide as 1.8 but a 18-55mm will allow you to shoot only at 5.0 aperture 
2. This difference itself is a huge deal. This means you can have the advantage of having faster shutter . Imagine shooting handheld at 1/15s with aperture f5.0 and then at 1/125s with aperture f1.8. This can be significant in low light conditions.

Any telephoto lens with focal length 200mm and above would be good. This is generic and affordable for most of amateur photographers. 

If you are willing to specialize in wildlife or willing to venture out in wildlife photography, can go for 150-600mm lens.

There are basically three fundamentals that you should follow for photography. Those are

1. Shutter Speed
2. Aperture
3. ISO / Film Speed

You can find a nice article on this fundamentals over here

But before that, you need to practice on Program Auto or Auto Mode (Creative Mode on Canon) for clicking in perfect composition. When I bought my camera, the sales guy suggested me to start with the only composition, rest you can master later. 

Here are some photo composition tips by Steve McCurry - The famous photographer

In any lens, elements are the specific lens pieces used and how are they arranged to obtain a particular feature, the arrangement is the group. Lenses pieces are usually arranged in groups. In lens, say if it is a zoom lens, it will have more number of elements and groups, where as for a prime lens, it will have less number of groups and elements. 

Usually it is termed as the more number of groups and elements, the better the quality but it is not always true. Lens quality also depends on the type of glass, design mechanism, manufacturing process of the lens. So for a particular focal length, a prime lens would be better than the zoom lens.

Prime lenses usually perform better than zoom lenses.