What is the difference between a concentration and a track within the major?
'When I use a word,' Humpty Dumpty said, in rather a scornful tone,
  'it means just what I choose it to mean — neither more nor less.'
  
  'The question is,' said Alice, 'whether you can make words mean so
  many different things.'
  
  'The question is,' said Humpty Dumpty, 'which is to be master — that's
  all.' 
  
  -Through the Looking Glass


"We don't have majors, you see..."

First, some institutions don't have "majors", they have "concentrations". This doesn't seem to be what you are interested in specifically, but I include it to avoid confusion. And it's also a good example of just how much definitions of these terms can vary between places.

Track

A "track" usually means there are two (or more) relatively independent paths of study within a single program (major), especially when the goal is to be ready for a certain end-goal (such as for a certain profession or field of research). These usually have suggested courses to take in a certain order, and while they may have a common core at the beginning, by later years people on different tracks will likely have completely different courses to satisfy their graduation requirements.

A very common use of track is to cater to those who are planning to go on to graduate school vs those who want to go directly into industry, or even to cater to different professional requirements. This is perhaps heavily influenced by many high schools in the US having "college track" and "job track" offerings/programs.

Focus

Similar to a track but with less differentiation between tracks, many institutions will offer a "focus". This usually means that all focus areas have most of the same basic courses and electives, but that just a few select higher-level courses and electives are in a certain direction and have maybe additional lower-level requirements. A Computer Information Systems program might offer a focus for software development vs networking/infrastructure, for instance. Focus options might only differ by 3-4 classes, and often have all other classes in common.

Concentration

A concentration (when it isn't a synonym for a major) is often just a more extensive and directed version of a focus. A concentration at an undergraduate level will usually have 4+ courses to satisfy a specific concentration, and often have additional special requirements. A psychology program might offer a general psychology major and a human services concentration for those who have a particular interest in being a clinician, for instance. Using my example programs, a concentration might have preset options for core classes (a choice to take a class in personality vs abnormal psychology is made for you if you want to concentrate in human services), electives are replaced with required classes or must be chosen from a restricted list, and some have special requirements for a final course or internship/residence experience.

So just at my one present institution the words concentration, track, and focus are all used and they all mean different (yet relatively consistent) things. Other colleges in the US often use similar definitions, but the range on this is pretty wide; as I pointed out above, some colleges use completely different meanings for these terms. 

Minor Details

I've heard some programs even replace the idea of a "focus" or "concentration" with having a minor program to be chosen, for instance. As a small taste of diversity even within a mid-size institution, over in our History department they offer a minor in Peace Studies, yet its requirements are not very different than a "concentration" offered in our Psychology department. 

Typical Physicists

This is to say nothing of our Physics department, which offers no less than five versions of a bachelor's degree - plus a pre-engineering two-year option and a tight relationship to Astronomy - and they don't call them a track, concentration, focus, or minor: it's an "emphasis"!

YMMV

My list here highlights meanings and differences within just a single mid-sized institution, and I've pointed out at least one place where the words mean something completely different, and shown a small taste of variation between programs. 

For any specific case of actually deciding what your department would offer, I'd suggest looking up a few hand-fulls of 'competing' departments from a variety of US institutions and comparing how they handle the officially listed tracks and meanings - I suspect they will be all over the place, but I think this general idea will be a common theme. It might also be wise to compare your department to others within your specific institution, under the assumption that there will be more interaction within campus than between far-flung US campuses (though your recruitment/admissions office might not agree with that).

If nothing else it will 'feel' more scientific and professional than handing out darts at a faculty meeting (and perhaps less dangerous).