What is an admitted insurer?

Admitted or standard insurers are regulated by the State while non-admitted (aka non-standard or Excess & Surplus Lines / E&S) insurers are not. Non-admitted therefore have a lot of freedom with rates and underwriting that the standard carriers don't have. Admitted insurers also participate in the State Guaranty Fund which pays claims when insurers become insolvent while non-admitted insurers do not. Those are the main differences.