If you are arriving from within the Schengen zone (all flights to/from Terminal 2) there is no passport control. Most EU countries are now part of Schengen (excluding the UK).
There are three signs at Passport Control: 1) Non-EU, 2) EU only and 3) EU Only but with priority for diplomatic staff and flight crews. Most nationalities do not require a visa to enter the country but you can check here. Compared to other EU airports, passport control can sometimes be quite slow (this is most likely to be the case if you arrive during peak times, from 10:00 a.m. to 2:00 p.m.). There are 12 passport gates but generally not all of them are attended at once. Overall, security at Prague Airport is tough but efficient.