What is the difference between WEAR and ADD?

[Pombokom]

Wear and Add seem interchangeable, but I'm sure there are distinctions too subtle for me to figure out on my own. I haven't found yet the place where this is simply explained.

[Lillith Hapmouche]

Both options attach something to your avatar. 

Wear replaces any attachment on the same attachment point.

Add adds it on top.

That's basically it.

[Rolig Loon]

Try a simple experiment:

1. Rez a prim, label it "Box A", and take it into your inventory.

2. Now, find Box A in inventory, right click on it, and select Attach To >> Head.

That's the same as ADDING the box to your head.  You should see it there now.

3. Detach the box.  It will remember that it is supposed to attach to your head next time you use it, so here's the REAL experiment:

4. Find Box A in your inventory again, right click on it, and select ADD.  It should attach to your head again, just as before.  No surprise.  So detach it again.

5. One last time .... Find Box A in your inventory, right click on it, and select WEAR. It will attach to your head OK but POOF!  Your hair is gone, replaced by the box.

Now you have to go digging through your inventory to figure out where you put your hair, so you can WEAR it to replace the box.

As Lilith says, ADD attaches an item to a specific spot on your body, regardless of what might already be there.  WEAR replaces what might already be there.  That's why newbies occasionally find themselves buck naked when they WEAR a piece of jewelry that is meant to attach at the same spot where they already have a dress attached.