From GuildWiki

Skills and attacks all have a certain range. A character trying to attack or activate a skill must be within range of the target before the attack begins or the skill starts to activate. In most cases, issuing such a command from out of range will have the character attempt to move to within range in order to perform the action.

Range chart

The testing circles at the Isle of the Nameless

A view closer to top-down

A color-coded view of ranges on the in game compass

¹ Some skills do not have a specific target but affect the entire party. Those skills will actually only affect party members in selection range.

² Projectile weapon ranges given here assume the target is at the same height as the attacker.

In some game updates, ArenaNet has referred to compass range which is the same as selection range.

Tips

• For a visual reference in the heat of battle, it is useful to note that the shadow cast by your character is roughly the same size as the innermost testing circle (i.e. adjacent to self).
• The automatic path-finding mechanisms direct your character closer than the intended ranged action strictly requires to succeed. For example, a player can issue an attack command from out of range and their character will run closer and attack. However, that character will be able to back up slightly and still attack if an attack command is issued from there.
• Height only factors into range calculations for projectiles, not including those from: spells, staves, or wands.

Disclaimer

The Guild Wars official website as well as the game manual for Guild Wars Factions contain a section on range information measured in feet. That information is inconsistent with the game (most notably that it claims touch range is much shorter than melee range and that flatbow range equals aggro range) and should be disregarded.