"An RT-PCR test, like the normal PCR test, uses swab samples from the nose and throat to find the Covid-19 virus. While an RT-PCR test will allow travellers into countries where a PCR test is an entry requirement, certain countries accept only RT-PCR tests."
Above was on one of the test clinic websites. Although it doesn't explain the scientific difference between the two, it does explain the importance of getting an RT - PCR test if required by the country you are travelling to. The Royal Thai Embassy London has stated that a RT-PCR test is required for entry into Thailand.