This does not have to be a perplexing problem for homeschoolers.
A "Carnegie Unit" is a measure of credit assigned to a certain level and amount of high school work completed. The word is interchangeable with "Credit".
One credit is typically given for a full year course. A 1/2 credit is typically given per semester. However, it is the amount of material that generally determines this in government schools. If you complete the work sooner, you get the credit!
If you are creating your own curriculum, you must log the hours of study. Usually, one full credit is given for yearly hours accumulating 120 - 180 hours of work. Typical for electives is 120-149 hours. Typical for college prep courses (such as math, english, science) is 150 hours per year and up. Also, science classes with LABS should approach the 180 hour mark. Its always better to do a little more, than not enough.
This is intended as a general help for you. As always, please check with your state to find your requirements.