You'll be able to customize their look and all their armor and weapons in the same way that you can customize your own look and gear. During your journeys in SWTOR you'll even get custom items for specific companions.
Every character, by the time they've finished the main story line, will have access to five companions in total. Every class has five different companions which are unlocked as you progress. Whether you choose to journey with a companion because of their special skills, or just because you like their company, it's totally up to you. Some players may even choose their main character's class based on the companions connected to a class, but the five different companions attached to each class represent one from each of the five skill sets (DPS, tank, heals, etc).