Congress can regulate commerce with foreign nations, Indian tribes, and among the states. The Supreme Court said that Congress could act under the Commerce clause in any of the following three situations:
1) regulating channels of interstate commerce, such as highways, waterways, the Internet
2) regulating the instrumentalities of interstate commerce and persons or things in interstate commerce
3) regulating economic activities (including INTRAstate activities) that have a rational basis on which the activity could have an substantial effect on interstate commerce
4) regulating non-economic INTRAstate activities IF Congress can show a direct substantial economic effect on interstate commerce (NOT cumulative effect).
This is basically ANYTHING involving two or more states or any transmission across state lines (including electricity, gas, TV, radio, etc.).
Congress does NOT have the power to COMPEL activity where there is none, even if the failure to undertake the activity affects interstate commerce.