A "circuit" judge refers to a judge, usually appointed or confirmed by the governor of a state, to hear cases at the Circuit Court level. An "associate" judge is simply an honorary title. An "associate" judge may be a retired judge, or an inactive judge who occasionally hears cases. An "associate" judge would still have to be qualified to hear cases at whatever level of the court system he is functioning in.

The practice of a circuit court judge was for them to ride around the countryside each year to hear cases.

The Chief Judge of the circuit in which he sits.

Federal Circuit Court of Appeals judges are appointed for life and can only be removed via impeachment.

If a "county" judge is also certified to sit as a "circuit" judge, then yes, they may do so when serving in their Circuit Judge capacity. However a judge certified ONLY as a county level judge may not normally preside at a state-level criminal trial.