First, if you import a character that has this achievement, you will unlock it by fast travelling to a new place. But, if you don't have it....
For this achievement, you will need to discover all named locations in the Borderlands.
To make things simple, you should have NO grayed out areas on any of your maps. When an area is completely discovered, you will see no areas grayed out and a clear border around the edges of the map.
Also keep in mind that you won't be able to unlock this until after beating the game and receiving a few additional side missions. I am not going to type an entire location guide, but instead list some commonly missed areas.
Now, on to some easily missed locations.
1) Southern Shelf -- Fast travel to southern shelf. From there, head left and enter a location called Southern Shelf - Bay. If you didn't do the side mission "Symbiosis" earlier in the game, you wouldn't have discovered this location yet. Make sure to uncover all of the grey areas as there are about 3 or 4 named locations in Southern Shelf - Bay
2) Eridium Blight -- After uncovering the whole map, you'll probably notice a large square in the middle of the map that has yet to be discovered. If you head out right from the fast travel station, you will come upon a tunnel entrance that is guarded by lasers. Follow the electrical wire up to the nearby tower and either shoot or lob a grenade up to the fuse box. You'll know where it is because that's where the wire ends and it shoots out some sparks. After that, run back down and jump down to the bottom. Ka-ching, location discovered.
3) Sawtooth Cauldron -- After clearing out the buzzard's nest (the big oil rig looking thing in the middle of the map) you need to jump off of the SW corner into a small camp. There will be trunk full of loot there as a reward.
4) Arid Nexus -- If you did the mission involving pump stations in co-op, there's a good chance you missed discovering one of them. Pump station #2 is very hard to spot if you don't know where to look. Look closely at the map and you'll see a small square that is right next to the pipeline. Catch a ride to it and take the lift down to discover it.
5) Caustic Caverns -- Once again, after exploring you'll probably notice a grey area in the middle of the map. This one takes a bit of work. You'll need to do the side mission "Lost Treasure" At the end of the mission you'll take an elevator ride to the top of the building and discover the location along with some loot!
6) Ore Chasm -- After completing the game, you will get a side mission called "Hyperion Circle of Slaughter". To get there, head to Eridium Blight once more and catch a ride up to Mal (the robot you may or may not have helped during an earlier side mission.) Once through the doors, you'll discover Ore Chasm
7) Terramorphus Peak -- After completing the game, you'll get a side mission called "You. Will. Die. (Seriously). Head to Thousand Cuts and follow the waypoint down some stairs and into a cave. You'll need 4 eridium to enter this location. Once inside, you'll automatically discover the location. I strongly advise against trying to fight Terramorphus alone. (Additional note: User Deathsangelz commented saying that they got the achievement without visiting here, so you may not need to discover this location.)
8) User Macfly77422 pointed out another location that can only be found by doing a side mission. You will need to get the battle of the gangs quest from Ellie in the Dust. Sometime during the mission you will need to travel to Holy Spirits (the bar) and you will head down to the basement. You won't be able to discover this location unless you take this side mission.
9) Friendship Gulag -- Located in the Dust if I'm not mistaken. There's a path up to it if you head out right a ways from Ellie's garage.
This list of locations is not comprehensive. I listed some locations that gave me trouble and figured others probably had similar issues. If there are locations that gave you trouble or you would like to see added to the list, please comment or PM me.