What kind of transformer this is?
It is a "power transformer" probably E-I laminations and split-bobbin (and likely a cheap grade of electrical steel that is not very efficient). 

The red wires are the primary (220VAC/50Hz) and the blue wires each have 12VAC RMS relative to the yellow wire (secondary is 24V centre-tapped). Voltage will be higher with a light load, perhaps considerably higher.  

Claimed current rating is 3A RMS (resistive load only). 

Double check that the resistance on the red wires is much more than that between the blue wires before plugging it in.