You can start with:
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{lmodern}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{p{3.5cm}cp{1.75cm}p{3cm}p{1.75cm}}
\toprule
\multicolumn{3}{c}{Field variables} & \multicolumn{2}{c}{Field equations}\\
\midrule
Name & Symbol & Number of Unknowns & Name & Number of equations \\
\midrule
Displacement (vector) & \textbf{u} & 3 & Conservation of Linear Momentum & 3 (3.2.9) \\
Strain (second-order symmetric tensor) & $\varepsilon$ & 6 & Kinematics & 6 (3.2.4) \\
\bottomrule
\end{tabular}
\end{document}

Update: Following Johanne_B and Barbara Beeton suggestions, better aligning and cmidrules are introduced.
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{lmodern}
\usepackage{booktabs}
\usepackage{array} % for \newcolumntype
\newcolumntype{v}[1]{%
>{\raggedright\hspace{0pt}\arraybackslash}p{#1}%
}
\begin{document}
\begin{tabular}{v{3.5cm}cv{1.75cm}v{3cm}v{1.75cm}}
\toprule
\multicolumn{3}{c}{Field variables} & \multicolumn{2}{c}{Field equations}\\
\cmidrule(r){1-3}\cmidrule(l){4-5}
Name & Symbol & Number of Unknowns & Name & Number of equations \\
\cmidrule(r){1-1}\cmidrule(rl){2-2}\cmidrule(rl){3-3}\cmidrule(rl){4-4}\cmidrule(l){5-5}
Displacement (vector) & \textbf{u} & 3 & Conservation of Linear Momentum & 3 (3.2.9) \\
Strain (second-order symmetric tensor) & $\varepsilon$ & 6 & Kinematics & 6 (3.2.4) \\
\bottomrule
\end{tabular}
\end{document}

multicolumnand thebooktabspackage. – Johannes_B Jul 01 '15 at 13:26