In parts of my woods, they are small, thin, lax and weak from too much shade. Since they root if the stems touch ground, as well as seed copiously and send up shoots from shallow roots, they quickly form virtually impenetrable thickets if they get enough sun to be robust. Those on the multiflora rose are one of the keys to identifying it because the margins are feathery or comb-like. A modified stem, always originating at a node, with nodes and internodes along the length of the thorn. Multiflora rose stems are armed with two kinds of thorns.
Some roses have all one type and some have different shapes.
They can be straight or curve down or up - which is rare. The most difficult to encounter are the hooked thorns, which seem to me to be mostly on older stems on the multiflora rose. They grab and don't easily let go. Unless we are botanists, we speak of roses are as having thorns, but the correct term for them is prickles. Being accurate in this is not an easy task as my vision of a prickle is something small and almost soft until you hit the pointed end.