Monitors are the final piece of the puzzle that can adversely affect an optimal system as what you are hearing is ultimately the end result of everything achieved in the production process. You can have a state-of-the-art console, DAW, audio interface and computer but if you have sub-standard monitoring systems, all of the research and funding injected into the other parts of a functioning system are irrelevant.
In general, there are 2 specific typed of monitors; active and passive.
Active monitors do not require an amplifier as part of the system as the amplifiers are independently built into the cabinet of the speakers hence why active monitors are physically heavier than passive. Passive monitors require an amplifier to feed the signal to them.
Active vs. passive monitors is once again a subjective decision. One system is rarely favoured over another upon initial research. The only thing that may sway a decision is the convenience of a 2-part system (2 active monitors) over a 3-part system (2 passive monitors, plus an amplifier. Professional studios often have plenty of room for an amplifier, thus selecting a speaker systems relies solely on personal preference.
No two brands of monitors will sound the same. There will always be slight differences in frequency response and range. The circuitry, the design, the shape, the size and the quality of the materials used are all contributing factors to how a speaker sounds in response to the signal it is fed. Slight coloration and dim frequency absences will occur throughout the audible frequency spectrum. The location of these inconsistencies again depends largely on the factors mentioned above.
Choosing a speaker system with the flattest frequency response is not always the best base-line to work from. The colouration of a monitoring system will be slightly altered based upon the room design and treatment of the space in which they are set. Therefore, alterations are ultimately made to the response of our ears.
When mixing and mastering, engineers will often flip to different monitors in an attempt to understand the translation of one monitor to another. Understanding this translation is largely what sets apart the good engineers from great ones.
The location of your speakers within a room is crucial to their reliability. In an extreme sense, listening to speakers that are facing the wall versus listening to them directly will of course have a drastic effect on what you hear. Monitors should be set up symmetrically in a room to avoid the potential for reflections reaching the ear at different times. The most ideal setup for monitors and the listener will create an equilateral triangle meaning, the “mix position” is equally as far away from both monitors as both monitors are from one another.