What do the different symbols on the map represent

It seems they are generally points of interest related to Nord mythology. I methodically went to each X on the map, and here's what I found:

Guardian/standing stones

1. Northeast of Markarth: The lover stone
2. Northwest of Solitude: The steed stone
3. East of Morthal: The lord stone
4. Northwest of Morthal: The apprentice stone
5. West of Winterhold: The tower stone
6. East of Whiterun: The ritual stone
7. Eastmarch: The Atronach stone
8. South southeast of Riften: The shadow stone
9. Lake Ilinalta: The lady stone
10. (11.) (12.) Near Riverwood: The mage, warrior, and thief stones

The Serpent stone isn't marked on the map because, according to Nordic culture, the Serpent is the symbol of evil and is waging a constant war against the three guardians, those being the Thief, Mage, and Warrior stones, and those guardians' charges, those being the Lover, Steed, Lord, Apprentice, Tower, Ritual, Atronach, Shadow, and Lady stones.

Other points of interest

• Southwest of Rorikstead: Valthume
• North of Markarth: Ragnvald
• West of Karthwasten: Shrine to Peryite
• West northwest of Dragon Bridge: Volskygge
• West southwest of Solitude: The Statue of Meridia
• Southeast of Morfal: Labyrinthian
• Southwest of Winterhold: Alftand
• Southwest of Dawnstar: Mzinchaleft (I think: can't find anything else around here)
• West of Dawnstar: High Gate ruins
• South of Lake Jorgrim: Irkngthand
• The Velnontht Mountains (Northern): Mzulft (I think)
• The Velnontht Mountains (Southern): Kargrenzel
• South southwest of Riften: Forelhost
• West of Lake Honrich: Avanchnzel

These symbols are called Shadowmarks.

Glossary of Shadowmarks (All of the below comes from the in-game book, "Shadowmarks")

"The Guild"

This is the symbol for the Guild. This means the place is as safe as the Flagon's cistern. If you see this shadowmark, someone from the Guild is nearby for certain.

"Safe"

We usually leave this shadowmark when we've scouted and found a safe way around something, a hallway without traps or maybe a house that's already cleared out. If you see one of these, head the way it's pointing and you'll be fine.

"Danger"

If you see this shadowmark, head the other way or take your life in your own hands. It means there's something ahead or beyond that door that wants to turn you inside-out.

"Escape Route"

Now, on the rare occasion (it better be a rare occasion if you want to work in the Guild) that you find yourself in jail, look for this little beauty. You see this shadowmark and escape is just a few steps away.

"Protected"

We put these shadowmarks on places we don't want you to go. As in stay out of there or there's going to be a boot up the backside. These people are under the Guild's protection and should never be robbed or assaulted.

"Fence"

This should quickly become your favorite shadowmark. The person near this mark will buy your... hard-earned stolen goods for a fair price.

"Thieves' Cache"

Who says we only take and never give back? If you find this shadowmark near a chest or maybe a hollowed log, you're in for a surprise... a gift from the Guild for the thief in the field. Whoever said membership didn't have its privileges?

"Loot"

There's something near this shadowmark worth stealing. Saves you from breaking into a place only to find the people don't even have two septim to rub together.

"Empty"

The opposite of the Loot Shadowmark. Pass on this place, there's nothing inside.