Many people don't have a clue:

Just over half (55%) of Americans can correctly identify the three branches of government.
−-More than one in five (22%) believe the three branches of government are the Republican, Democrat, and Independent branches.
−-A full 16% of respondents believe the three branches of government to be local, state, and federal.

Less than half (48%) of Americans can correctly identify the meaning of the concept of separation of powers.
−-However, almost two-thirds (63%) report being very or somewhat familiar with the separation of powers principle.
−-82% feel that the separation of powers is important, while only 5% report that it is not important or they do not know what it means.