A 555 is used as an oscillator, optimally around 15.76KHz (in North America). Power pins on the chip (pin 1 to ground, pin 8 to +12V) are not shown on the diagram. The output from the 555 is then switched by PNP transistor Q1 and by the large 2N3055 NPN power transistor. Q2 must be adequately heatsinked as well since it may well switch over an amp during operation. Output terminals are then connected to a primary winding of the flyback. The average flyback has up to ten primary terminals and one secondary (the secondary identified as the thick, insulated, wire protruding from the top of the unit).

Source:
http://technology.niagarac.on.ca/staff/mcsele/lasers/HighVoltage.html