Rachel, generally I always considered the burn rate to be those fixed costs required to operate at minimal or no sales, i.e.; rent, heat light & power, interest etc. It's your "NUT".
Cost of Goods Sold is cost of the raw products that are used to create the finished product you sell. For example the cost for the rubber and steel to a tire manufacturer would be COGS. This is normally a fixed % of sales. Margin is usually that % remaining after deducting COGS % from Sales (which is 100%)
Sales (100%)- COGS%= Gross Margin%
Sales $ - COGS $ = Gross Profit $ Then Operating Expenses are deducted to determine Profit
This is of course in its simplist terms. I'm no CPA.