Non-monochromatic (multi-wavelength?) lasers
There are a number of answers to your question here and to write them all would take some time, but I'll try to answer some of them briefly and enough to satisfy the topic.
When a laser is marketed as a $x$nm laser it is centered around that wavelength, with a (most likely) symmetric spectrum around this point. To understand where the linewidth, as the width of the emission peak is referred to as, comes from we need to go through some simple quantum physics.
A laser functions through exciting electrons in a 2, 3, 4 or even higher level system, where they will eventually fall down to lower energy states. If these electrons are stimulated enough, there will be more electrons in higher states than in lower states. This means that if we insert some seed photons into this element, they will stimulate emission from this gain medium, and the emission will be larger than the absorption. This is the condition for lasing, that the gain overcomes the losses.
The electrons are said to be in energy states, and in quantum mechanics these energy levels are split into even finer energy levels centered around that one level. Thus, the electron can have a number of different, however close, energy levels. This means that electrons of not just one wavelength are being stimulated.
This is oe source of the finite linewidth of the laser. Another source which may limit the linewidth is the cavity making up the laser. I will not go into the makings of a laser in detail here and all the equations but the design of the cavity affects the linewidth as well as the gain profile of the laser medium.
Lastly, a laser linewidth can be affected by the environment. Atoms and molecules at higher temperatures for example broadens the distribution of wavelengths on each energy level and thus thi will also broaden the linewidth of the laser. Some external sources for this broadening is referred to as natural-, Doppler- and pressure-broadening.
With all this said, it is possible to design a laser given some spectral specifications. The center wavelength can be chosen if a lasing medium exists for this wavelength, linewidth can be tuned. Something i did not go over in this text here is transverse and longitudinal laser modes, but essentially even a laser linewidth is made up of even finer linewidths making up the complete linewidth.
I've not yet heard of a laser with more than one center wavelength. Modelocked lasers are laser with extremely broad linewidths where the modes in the lasers are locked with regards to each other and thus they create a pulsed behaviour with the energy contained.