Reflectors, or reflector telescopes, are telescopes that use reflective mirrors as objectives to generate images.  Light is reflected from the primary mirror onto a secondary mirror that will lead the light to an eyepiece so the image can be seen.

Large primary mirrors are much cheaper to make than large lenses and hence reflectors typically come with large apertures then refractors.  This makes them better to observe dimmer distant objects like deep sky objects.  Reflectors also produce slightly less sharp images which makes them less favorable to observe tiny bright objects like double stars or planets.

As light is bounced around a few times within the telescope tube, reflectors for the same focal length can be packed into shorter tubes than refractors.

Cooling

When bringing out a reflector telescope from a warmer inside into the cooler night air, the difference of temperature inside the tube compared to the cooler outside air blurs images.  In order to minimize this effect, reflector telescopes should be brought outside a good amount of time before starting observations for the air inside the telescope to cool down.  Small or medium-sized telescopes might have to cool down for 20 minutes.  Larger telescopes easily need up to 40 minutes or more to cool down.

Comatic Aberration or Coma

Due to imperfections in convex mirrors, point-like objects like stars get distorted.  This effect is generated by slight differences in magnification and is especially noticeable at the edges of an image.  Comatic aberration seems to add a comet-like tail to stars which is also where the name ‘coma’ comes from.

Coma correctors are special lenses that can be used on a reflector telescope to reduce coma.

Collimation

The mirrors in a reflector are not attached to the telescope like the lenses of a refractor.  They can and will move around slightly over time.  This results in blurry images.  Having the mirrors perfectly aligned is therefore of the upmost importance.  Collimation is the process of aligning the telescope mirrors.  Unlike refractors, reflectors need to be collimated once in a while to keep and this adds a little to your setup time.

Now if you use a laser collimator to align your telescope mirrors, make sure the laser collimator is collimated itself.