Fire Wall is just what it says- a wall of fire. When cast, it spreads out in a line perpendicular to the point clicked on, burning all those that walk through. The width and damage of the skill increase as more skill points are invested in it.
A critical note:Since a patching some time ago (source please), this skill has received a cool-down time to prevent clever players from spamming a whole room full of them. While this may or may not be bad news to you, the good news is that it also has increased damage now.
This skill, which allows the Sorceress to raise a barrier of flame to block her flanks from attack, gives her and her allies a tactical advantage. Any creatures unwitting enough to attempt to cross the barrier will feel the full force of these flames as they advance to their ruin.
↑Diablo II Manual, page 86. Blizzard Entertainment, 2000.