What do the different symbols on the map represent?

What we've got definitely isn't comprehensive, and portions may be too specific:

• = Settlement

• = House

• = Wheat mill

• = Lumber Mill

• = Farm

• = Stable

• = Port

• = Mine

• = Shack

• = Lighthouse

• = Imperial Army Camp

• = Stormcloak Camp

• = Nord Tower

• = Imperial Tower

• = Fort

• = Dragon + Word Wall

• = Standing stone

• = Barrow/Tomb

• = Surface Encounter

• = Giant Camp

• = Cave

• = Daedric Shrine

• = Dwemer Ruins

• = Camp

• = Orc Stronghold

• = Ship

• = Pond/Clearing

• = Grove

• = Mountain Pass

• = Generic point of interest. (Guild halls, some quest encounters, ect...)

• = Quest Marker - The quest is in the same zone as you.

• = Quest Marker w/ door - you must pass a loading screen to get to this destination.

In addition, each city have a unique emblem, as do each palace.

(Images from the UESP)