Market structure is the sequence of significant highs and lows, and the direction that follows from it. Upward structure: each next high above the previous one and each low above the previous one. Downward mirrors it.
That sounds like a definition of a trend, but it is not the same thing, and the difference is practical.
How it differs from a trend
"The trend is up" is an impression of a picture. Two people look at one chart and disagree, and the argument goes nowhere: there is no criterion.
Structure supplies one. While the reference low is intact the direction counts as upward — regardless of how the chart looks to the eye. The reference low breaks on a close and the direction has changed. The decision follows a rule, not a feeling.
How it differs from a trendline
A trendline is diagonal, drawn tangent to lows or highs. It depends on which points a person chose: move one and the line is different.
Structure is horizontal by nature: it is about the levels of specific extremes, not about slope. A trendline break and a break of structure are different events, and they routinely fail to coincide.
What exactly is visible on the chart
Three things, all checkable:
Extremes. Turning points that passed the chosen significance bar.
Reference levels. The latest high and low against which direction is measured.
Events. A reference level broken on a close, or pierced by a wick and recovered — different events, and telling them apart holds the whole markup together.
Where people get confused
Not saying at what scale. "Structure is broken" without a degree is an unfinished sentence: at the fine degree it breaks on every pullback.
Confusing a pierce with a break. Treat every touch of a level as a break and structure will change several times a day.
Expecting structure to predict continuation. It records a state, not a forecast: "direction is upward" means "the reference low is still intact", and nothing more.
What it is for
As a coordinate system for everything else. Zones are looked for before breaks, liquidity is tied to extremes, the range is measured from reference points. Without structure the other concepts hang in the air — there is nothing to mark and nothing to measure from.