Hi, under criminality they ensure that you are not booked for any crime from the countries where you lived for more than 6 months after the age of 18. So they counter check these police clearance certificates with the issuing authorities.
Under security check its like frisking you, shaking you up and down to see if anything thats poses as a risk falls off from you. They check with interpol, concerned embassies, check with all the databases available both nationally and internationally. They cross check if applicant is directly or indirectly involved in any social unrest, illegal trade, a suspect underwatch list, a supporter or activist of terrorism etc. Security check takes a bit of time unless you are applying from one of the 5 eye nations.