Viewers that mark the trader show how far Haldor is from your spawn. A trader on the far side of the map is a real cost, because most players want to reach him early.
Seeds are case sensitive and only the first ten characters count, so a longer string is cut off. Copy the seed instead of retyping it. Swapping 0 for O or l for I gives you a different world.
Check it in a viewer that matches your game version
World generation can change in major updates, so a seed that placed things well on an older patch may look different after 1.0. Preview the seed in a viewer that supports your version before you commit a long-term world to it.
A seed viewer generates the whole world from the seed in your browser, so you can see the starting area and the biome distribution before anyone has explored a single tile. Most viewers also show spawn details.
Viewers that mark the trader show how far Haldor is from your spawn. A trader on the far side of the map is a real cost, because most players want to reach him early.
Each boss has an altar in its own biome, and a viewer that marks them shows how far your later biomes are. The order is Meadows (Eikthyr), Black Forest (The Elder), Swamp (Bonemass), Mountain (Moder), Plains (Yagluth), Mistlands (The Queen), Ashlands (Fader) and Deep North (Kall Fimbulbringer).
Many viewers also mark crypts and dungeons and show where resources are. Check what sits within reach of the places where you plan to build before you commit to a base.
The Deep North generates only in areas nobody has explored. An old world will not produce the finished biome everywhere, and Iron Gate recommends a fresh world for the complete 1.0 experience. A brand-new world generates the Deep North fresh.
On the world creation screen you can type the seed you want or leave it to the game to generate a random one. On a dedicated server, set the seed when you create the world.
A Valheim world is a pair of files, WorldName.db and WorldName.fwl. Copy both while the server is stopped, and take the backup before an update changes the world.
You can check a seed in Valheim Map World, Valheim Tools or the Valheim Wiki seed map. The wiki's map generates the world in your browser, marks biomes up to the Deep North, boss altars, dungeons and resources, lets you add pins and routes, and gives you a link to share the map. I help make that wiki.