A table or figure stays where it is in the source: a page has no bottom for it to float to. Its caption is numbered and spaced as the document class sets it.
A table or picture whose width follows the column, such as a tabular* of width \linewidth or an \includegraphics with width=\linewidth, follows the reader’s column too. The build runs TEX at several widths, as for displays, and records how each box changes with the width.
Table 1: The inner planets: mass inkg, radius in km, length of the day in hours.
Planet Mass Radius Day Mercury 0.330 2440 4222.6 Venus 4.87 6052 2802.0 Earth 5.97 6371 24.0 Mars 0.642 3390 24.7
\begin{table}[h]
\caption{The inner planets: mass in $10^{24}$\,kg, radius in km, length
of the day in hours.}\begin{tabular*}{\linewidth}{@{\extracolsep{\fill}}lrrr@{}}\hlinePlanet & Mass & Radius & Day \\\hlineMercury & 0.330 & 2\,440 & 4\,222.6 \\Venus & 4.87 & 6\,052 & 2\,802.0 \\Earth & 5.97 & 6\,371 & 24.0 \\Mars & 0.642 & 3\,390 & 24.7 \\\hline\end{tabular*}\end{table}
The space between the columns of this table comes from \extracolsep{\fill}, so it grows and shrinks with the column. Below the table’s natural width it stops shrinking, and the table scrolls sideways instead.
Figure 1: A sine-Gordon kink at rest and moving at: the moving kink is narrower by the factor.
\begin{figure}[h]
\centering\includegraphics[width=\linewidth]{figures/soliton.pdf}\caption{A sine-Gordon kink at rest and moving at $v =0.8$: the moving
kink is narrower by the factor $\sqrt{1- v^2}$.}\end{figure}
A picture scaled to the column scales with it, height included. Pictures are drawn as inline SVG, so a scaled picture stays sharp.