Square bar is solid square steel, sized as edge x edge. Common gate-trade sizes are 10x10, 12x12, 16x16, 20x20, and 25x25 mm. Round bar is sized by diameter (10, 12, 16, 20, 25 mm).
Square bar is the traditional picket for steel-picket fences, a classic baluster for stair balustrades, and the standard for forged decorative work like scroll-and-spear panels. Solid bar costs more per metre than tube of equivalent size because you're paying for solid steel, but it gives a more substantial visual weight to a finished panel.
For structural applications (gate frames, posts), tube wins on weight-for-stiffness. Solid bar earns its keep in pickets and decorative components where the look or feel of solidity matters.