Transformers come in two general types: electronic and magnetic. An electronic one will be smaller and lighter, but can produce some electical noise. A magnetic one is essentially just a big 'normal' transformer. Each of them can be used to dim lamps attached, but the type of dimmer MUST be matched to the type of transformer, or you could end up with either a fire or a very short-lived dimmer. The biggest thing is to ensure you get a transformer rated for the wattage of the lamps you want to use. Too big is going to produce excess heat, and waste energy. Then, the second thing is ensure the output voltage aligns with the bulbs you choose (lower voltages require heavier wires because of higher amperage).