Fixed – Fixed downlights are the most basic and by far the most popular type of downlights and they are designed to point directly down without the ability to adjust. They are often times the most affordable and used in general purpose applications.
Adjustable - Adjustable downlights allow you to direct the light by rotating the light or adjusting the light at an angle.  These types of downlights offer increased flexibility and will normally cost a few dollars more.
Outdoor Lighting – Outdoor lighting can be rated up to IP67 (depending on the type selected) in order to be water resistant and used in many different outdoor applications.  They can be mounted into the structure, bollard or used with some type of mounting stake if needed.
Round & Square – Downlights are offered in both round and square versions and are available in either a painted, metal or custom color finish.
Multiples – These types of downlights usually come with either two, three or four fixtures which are mounted into the same housing to offer increased lumen output and a more modern design.  
Trimless Downlights – The trimless downlights offer a very modern and more minimal, seamless look which are very popular with many high end homes and commercial spaces.  They are mounted into the ceiling and incorporated with a finish kit which is then plastered into the ceiling for a sleek look.
Color Rendering Index (CRI) - is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source.
Color Temperature - of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Color temperature is a characteristic of visible light that has important applications in lighting, photography and other fields.
Lumen Output - The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source.  The required lumen output will depend significantly on various factors and it is important to ensure you have selected fixtures that offer sufficient lighting for your project.
Beam Spreads – lighting of all kinds has its own unique beam angle and it is important to understand how and what type of angle you need for each project.  A beam angle is a measurement (listed in degree units) of how the light is distributed from the light source.   Beam angles are offered in narrow (10 / 15 degree) and they are offered in wider versions of 25°, 40°, 60°, etc.  If you select a wider beam angle (60 degrees or more) then they will typically fall in the category known as “floods”.  The type of beam selected will depend on the various factors such as the type of activity that is to be performed in the room, the ceiling height, will the light need to be adjusted in the future or is the light serving as general purposes lighting only, etc.