It still amazes me that in summer I see motorcyclists riding their machines dressed in shorts. Certainly they're cool (and I'm referring to temperature here) but, once again, what happens if they fall off? Even denim jeans don't offer much protection I'm afraid (unless you've sent them away to have Kevlar panels sewn in!). They're also not windproof, waterproof, or even slightly abrasion resistant.
Quality is important. Choose something that will be flexible, resistant to abrasion, and provide a degree of support. It should have padding on key areas such as knees to offer you protection from road burns. Good quality leather is generally regarded as being the most suitable material, although artificial materials containing Kevlar will also do the job.
What about "chaps"? Well, they're available in leather and textile and offer more protection than jeans. However, they only protect the legs and not the bottom, hip or groin.
Leather trousers can be expensive and heavy; but offer a much higher level of protection. Once again however, they are not waterproof so you will need some sort of over-trousers if you're going to be riding in the rain. They are also generally not as well ventilated as textiles so can be uncomfortable in hot weather. Not designed to be worn over normal trousers.
Textile trousers on the other hand can be worn over normal trousers, and are generally cheaper. Some styles include padding and armour.
Once you have spent your hard-earned cash on some decent gear, you need to look after it. Clean it regularly and treat it with a waterproofing compound like . I seem to be referring to this stuff frequently, but make no excuses for it; it's very good!)