What's the difference between perception and consciousness? Having read descriptions of perception and consciousness (two of the five skandhas), I still don't understand the difference.

Is there an important difference? What is the difference and why is it important?
In relation to the five aggregates, the consciousness aggregate solely refers to main minds, such as the eye-consciousness, the ear-consciousness, and so forth. This excludes mental factors. Mental factors are consciousnesses but they are not included within the category of consciousness aggregate. Except feelings and perception (or 'discrimination', etc.) mental factors are included in the category of compositional factors.

Because perception is a consciousness, it is clear and knowing. It means that it has the capacity of taking the aspect of the object it apprehends, and it knows/apprehends its object.

According to Mahayana Abhidharma, perception is one of the five omnipresent mental factors. It means that perception accompanies every single main mind. Here is an example: an eye-consciousness seeing blue takes the aspect of blue, and that aspect is apprehended by the perception factor that is in the retinue of the eye-consciousness as well. A main mind and its concomitant mental factors share five similarities that are explained by Yeshe Gyaltsen in his Necklace for Those of Clear Awareness Clearly Revealing the Modes of Minds and Mental Factors.

Regarding the entity of perception, the Compendium of Knowledge says:


  QUESTION: What is the defining characteristic of discrimination
  [Tib. 'du shes, literally, aggregation-knowing]?
  
  RESPONSE: It has the characteristic of knowing upon aggregation. It has the entity of
  apprehending the sign and apprehending the mark, through which one
  designates an expression to objects of perceptions, hearing,
  differentiation and knowledge.


Yeshe Gyaltsen explains further:


  Just as it has been said above, it is a knower that, upon the
  aggregation of the three – object, sense power, and primary
  consciousness – apprehends the uncommon sign of an object. Also, A
  Discussion of the Five Aggregates says:


Later in the text, he explains two division:


  
  The apprehension of a sign with regard to an object means to
  apprehend it through individually distinguishing the object's
  particularities, such as blue, yellow, and so forth.
  The
  apprehension of a sign with regard to a convention means to apprehend
  it through individually distinguishing the convention's
  particularities, as when one thinks, "This is a man; that is a woman."