when you fill up your gas tank at a service station you are completing a process that started with an oil exploration company locating an oil deposit continued with a drilling company extracting crude oil and then went through many stages of transportation refining and distribution before it got to your gas tank a number of people conducted business to get it there 
commerce refers specifically to the exchange of products or services between two or more parties in the above example you engaged in commerce when you paid to fill up your gas tank along the way there were other examples of commercial activity for example the crude oil was sold in bulk to one or more oil companies that was a commercial transaction as well the distinction between commerce and trade is pretty fine both are the direct exchange of goods and services for something of value between two parties in modern times something of value means money however there are some differences in their usage regulating commerce