Although an orthodontist is a kind of dentist, it would do one well to not confuse the two, as they are quite different from each other and unique. The difference between a general dentist and an orthodontist is that an orthodontist has specialised in the alignment of the teeth and in trying to get them to properly aligned, thus alleviating any symptoms due to the malocclusion of teeth, and preventing future problems from occurring.
More than a general dentist?
Technically speaking, an orthodontist is a general dentist that has had some extra training and can make, put on and activate braces so that they realign your teeth. This means that they can, technically speaking, provide all of the treatments that general dentist can. The truth is, an orthodontist is usually very busy putting on braces and doing orthodontic work, and thus will usually (but not always) not have the proper routine in doing the work of a general dentist, who deals only with these kinds of problems. A general dentist will have much more routine at providing fillings, root canals and other restorative work, and many times an orthodontist will not want to give these kinds of treatments at all.
Where to go
If you want a tooth filling, you should go to a general dentist, although tooth fillings can be handled by an orthodontist. If you are preparing for braces and the orthodontist notices that you have a cavity or a pre-cavity, they should be more than able to provide you with a tooth filling, especially as it may be within the course of the orthodontic treatment to first restore the tooth surfaces so as to minimise the chances of tooth decay during orthodontic treatment. If you are looking for a root canal though, have a general dentist look at it, as an orthodontist probably cannot do a good job. An orthodontist is perfectly able to give you a preliminary hygiene session necessary to being orthodontic treatment, but will usually leave these kinds of tasks to an oral hygienist. You should go to an orthodontist only for orthodontic treatment.