I find this website to be the best to search for timetables -
http://rozklad-pkp.pl/en/
There are a number of categories as stated above
EIP - The nicest and fastest services. Operated by new Pendolino trains. 'Much' more expensive than other trains in relative terms.
EIC - Express Intercity. Not sure if there are many left as these are being replaced by EIP. Basically IC, but slightly faster.
IC - Intercity. Fairly nice, modern stock. A good option if it's available.
TLK - The cheapest intercity trains. I've been on lots of these and the stock varies a lot. Some are reasonable, if a bit dated. Some are ancient and dirty, or 'cheap and cheerful' as someone pointed out above :P
All trains have to be booked with reservations and this has been the case for quite sometime. I found I could often book trains on the day, but don't rely on this. Torun is an easy weekend away from Warsaw and I imagine Sunday evening trains could be busy.
You can buy a ticket onboard only if you board from an unmanned station, or you have to pay a fine. The conductor's compartment is normally at the front of the train.
There are also regional trains, which strangely often go very long distances. Some are operated by the national company PR but others are operated by regional governments. These don't have reservations and tend to be fractionally cheaper than TLK trains. Trains operated by PR tend to be old, run-down and pretty dirty. Other regional operators have newer, modern trains which are much nicer. They only have second class.
On regional trains you can buy tickets onboard, but you have to go and find the conductor. If he/ she finds you, then you have to pay a fairly substantial fine.