Floorcraft, or dance floor etiquette, is an informal collection of protocols and guidelines that have evolved to maximize everyone's enjoyment.
Good floorcraft allows the couple to navigate smoothly around the dance floor, staying in "Line of Dance" (moving counterclockwise around the dance floor) while avoiding collisions with walls, furniture, or other dancers. With some exceptions, floorcraft is the responsibility of the leader. This allows the follower to "zone out" and just enjoy the embrace and the music, trusting her follower to take care of her.
The main goal of floorcraft and dancefloor etiquette is to avoid injury - to the follower, to other dancers, and to walls and furniture and potted plants. This requires that the leader be paying full attention to his surroundings and to other dancers and their styles of dancing - whether they are controlled and disciplined, or whether they are wild and crazy.
The second goal is to keep traffic moving around line-of-dance. This means not stopping and blocking other dancers behind you if there is an empty space in front of you.
As you progress CCW around the floor in line-of-dance, the most strategic place to dance is near the wall, close enough that no one has space to get into your blind-spot by passing you on the right.
Stay in single-file if you can. At a crowded venue, it will naturally happen that couples will migrate into several "rings" or "lanes" of single-file dancers. Try not to switch lanes to pass slower couples, unless they appear to be clueless and a traffic jam of dancers is building up behind you.
Is is customary to not talk at all while dancing at a milonga. If you want to talk or teach or discuss a step, it is polite to wait until you are off the dance floor so you don't disturb other dancers. At practicas, as mentioned above, this protocol is relaxed, and it's OK to talk, to work on your steps, or to discuss technique while on the dance floor.
Entering the dance floor while other dancers are dancing is like merging onto the freeway: wait for an opening, make eye contact if you can, and move in gracefully without disturbing the other dancers, rather than forcing your way into the line of dance.