You can keep track of your companion influence in this window. Companions can also be sent on gathering and crafting missions based on what Crew Skills your current character has learned. For the most part, all companions are equally effective in all roles, save for few exceptions - but this is a topic for another guide.
BioWare has tweaked and rebalanced how companions work several times over the years, and as of 6.2.1 all companions can perform all three roles: healing, tanking and damage dealing.
You can have one companion with you at all times. By default, every class gains 5 companions throughout chapters 1-3 of their class story, and there are additional opportunities to gain companions in further chapters, through promotions, but completing achievements or by purchasing them on the Cartel Market. In broad strokes the system should be familiar with anyone who isn't new to RPGs.
