Flax is the plant. Linen is the fabric made from it. It's a small distinction but a useful one, especially when you're reading a care label or a supplier's spec sheet.
The flax plant (Linum usitatissimum) produces long fibers inside its stalk. Those fibers are separated out through a process called retting, traditionally done by leaving harvested flax in a field to be broken down naturally by dew and bacteria, then spun into yarn and woven into linen fabric.
The quality of the finished linen depends heavily on the length and consistency of those fibers, which is why sourcing matters more than most people assume. Short, inconsistent fibers produce a rougher, weaker fabric prone to pilling. Long, well-processed fibers produce the smooth, durable linen that softens over time instead of breaking down.
This is also why "100% linen" on a label doesn't guarantee quality on its own, the fiber source and processing matter as much as the fiber type. It's a big part of why we're particular about who we buy from.
