what do we learn about the two types of police?

There is the thought police and the regular party police that watch and regulate day to day activities. The thought police are a special unit that investigate thought crime. If someone shows a propensity for certain thoughts (facial expressions, body language or an offhand remark) that defy party dogma they are investigated and usually "unpersoned" (killed) or re-educated. Being killed, by the way, would be preferable to the former. The regular party police simply watch and manage people. They monitor and report suspicious behaviour (that seems contrary to the Party's goals, to the appropriate ministry.