Flexbox and Grid are both modern CSS layout systems, but they solve different problems. The core distinction: dimensionality.

The fundamental difference

• Flexbox is one-dimensional — it lays items out along a single axis: a row or a column. Wrapping creates multiple lines, but each line is still managed independently.
• Grid is two-dimensional — it controls rows and columns simultaneously, as a true grid where items can span and align in both directions at once.

Content-driven vs. layout-driven

• Flexbox is content-driven. You give it items and it distributes space based on their content and flex properties. You don't define the structure up front.
• Grid is layout-driven. You define the grid structure (grid-template-columns/rows) first, then place items into it.

When to use which

They compose — it's not either/or

The common real-world pattern: Grid for the macro layout, Flexbox for the micro.

.page {
  display: grid;
  grid-template-columns: 240px 1fr;
  grid-template-rows: auto 1fr auto;
}
.card {
  display: flex;          /* Grid placed the card; Flexbox lays out its insides */
  justify-content: space-between;
  align-items: center;
}

Quick syntax reference

/* Flexbox */
.flex { display: flex; justify-content: center; align-items: center; gap: 1rem; }

/* Grid */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
  gap: 1rem;
}

repeat(auto-fill, minmax(200px, 1fr)) is the famous responsive-grid one-liner — no media queries needed.

Senior framing

The senior answer resists "Grid is newer/better." It's 1D vs 2D, content-driven vs layout-driven, and the mature take is that they compose: reach for Grid when you're describing a structure, Flexbox when you're distributing items along a line. Knowing the minmax/auto-fill responsive idiom and gap (now supported in both) signals current knowledge.