There are thirty-one different species of sentient creatures in the galaxy, though not all appear in the books, or at least they don't yet. The ones so-far mentioned are listed below with a brief description.
Aborigines: An iron-age civilisation of hairy bipeds, discovered and protected by the Centaurs.
Bastards (or Bs, if you're being polite): Aggressive grey reptiliods, the males are about about 2.5 metres tall. The females are a bit smaller. The name should give you a clue as to their general demeanour.
Centaurs (Orange): Six limbed, black creatures with an orange mane and three eyes, a larger central one and two smaller ones on either side. They speak through their nose. They form permanent pair-bonds, when widowed the survivor often commits suicide.
Centaurs (Lilac): A different species of Centaur with a lilac mane, but have different social structure to the orange Centaurs, in that they sometimes divorce each other, which the oranges consider abnormal.
Chipmunks: Human-sized rodent creatures, bipeds and look mammalian but lay eggs. They look a lot like giant versions of the Earth creatures.
Dans: Like six-limbed geckos, their favourite narcotic, streng root is deadly to most other species.
Emus: Large blue bipeds. They look a bit like deformed emus or ostriches, but have larger heads and arms instead of wings and fur instead of feathers.
Gralls: Aggressive lion-like creatures. Their blood is blue as it is a form of haemocyanin.
Insectoids: Large insect-like creatures but with four segments (instead of three) and eight legs. Think of lace-wings.
Multipedes: Think of the front of a millipede and you'll have a good idea of what they look like. They are the only other species that uses base 10 as a counting system, as they have ten legs.
Octobears: Giant aggressive eight-limbed bears.
Rocks: Like giant grazing rocks. They evolved on a planet with 3x Earth's gravity. Therefore they tend to colonise planets with strong gravity.
Terinths: Six-limbed squid-like creatures but terrestrial. They walk upright.
Über-slugs: Look like giant slugs (but have human-like eyes), very unaggressive. Communicate by changing their body colour, like squids on Earth do. As they can change shape and colour, they are very difficult to tell apart. They change sex, so using the correct pronoun can be a bit tricky. People just tend to use the default for their language (male in English).
Wasps: Giant Wasp-like creatures, evolved on the same planet as the Insectoids. They have the same general body-plan.