What type of certification an aircraft has matters – it determines how the aircraft must be maintained, where and how it can be flown, and how the hours flown in the aircraft can be logged.
Finnish registrations: Left is Certified Aircraft (OH-_ _ _), Center is Experimental (OH-X_ _), Right is an Ultralight OH-U_ _ _)
In Europe, EASA (The European Aviation Safety Agency) Type Certified Aircraft (CS-23, CS-VLA, CS-LSA) have the least amount of operating restrictions but are professionally built and maintained which can add to acquisition and running costs.
Experimental aircraft are usually a lot cheaper to buy and maintain, but they are seen by regulators to have lower safety standards, and hence have more restrictions on how they can be used. They cannot be used for commercial purposes and here in Europe cannot fly IFR.
Ultralights (aka Microlights) are not governed by EASA and fall under national legislation. They are usually factory built but have stringent wing load, stall speed and weight restrictions (traditionally 475.5 kg in Europe, but new regulation coming in 2021 will allow certification up to 600 kg). Travelling abroad with Ultralights can also require pre-approval from the states being visited.
Bang for buck Ultralights are awesome, but annoyingly the hours accumulated do not count towards a Single Engine Piston (SEP) rating. This may be a showstopper for those who are trying to build hours for a professional career.