Third Class Medical Certificate: necessary to exercise the privileges of a private pilot license or certificate. You can also exercise the privileges of a recreational pilot certificate, student pilot certificate, or flight instructor certificate with this medical certification. In the United States, it expires after 60 calendar months for someone under the age of forty years, or 24 calendar months for someone over forty.
Second Class Medical Certificate: necessary to exercise the privileges of a commercial pilot license or certificate. In the United States, it expires after 12 calendar months.