Doors keep people out of homes, businesses, rooms, and other places where where don't want people to enter without permission. When they are locked, the message is quite clear.
Conversely, doors (when they are unlocked and open freely) allow people to enter an otherwise private or semi-private space or one that might be temperature controlled or protected from the elements.
When doors (either exterior or interior) are wide open, the message is clear that anyone may enter or use the space.
Some doorways or access points don't even have a traditional door - using instead a cased opening (one that looks like a doorway but without any door slab attached in the space) or and archway. Both cased openings and archways can be quite wide and attractive. In may ways they really dress up a space.
As for those entries or access points - again exterior or interior - that have doors, they come in a variety of styles and sizes. Some open from the left, and some open from the right. Some open in, and some open out as one faces them.
Mention the word "door" to someone and they likely will think of a traditional door that is hinged along one side with a door knob or handle opposite the hinged side. Even here, though, there are solid core doors - all wood - that typically are 1-3/4" thick and hollow core (generally 1-3/8" thick) with a veneer facing on each side and a honeycomb of another material between them to provide some stiffness and shape.
Doors can be plain ("flush"), carved, raised panel, 3-panel, 6-panel, or other styles. They can have grills or windows in them also.
For interior or patio doors, there are many more styles - sliding glass, sliding barn door styles, bi-fold (louvered, partially-louvered, flush, or paneled), by-pass, or pocket. Due to the variety of door styles, personalization and customization to individual tastes and needs is quite doable.
Three important reminders about doors: (1) make sure there is enough access room (generally 24" or so) to approach the door and open it without adjacent walls or furnishings interfering with that motion, (2) make sure they are the widest possible for people needing to use the space plus anyone who might be visiting (generally a minimum of 36"), and (3) be aware of how the opening of hinged or bi-fold doors (or those that remain in the open position after being opened) may interfere with passageways or other doorways.
Actually, there's quite a bit to consider about doors, and this isn't even all of it.