Summary

Displacement current $I_d=\epsilon_0\,d\Phi_E/dt$ is Maxwell's addition to Ampère's circuital law, needed because a changing electric flux (e.g. between a charging capacitor's plates) produces a magnetic field even where no conduction current flows; the corrected Ampère-Maxwell law is $\oint\vec{B}\cdot d\vec{l}=\mu_0(I_c+I_d)$.

Sources: an ACCELERATING (typically oscillating) electric charge radiates an electromagnetic wave; the changing electric and magnetic fields regenerate each other indefinitely (changing $E$ → magnetic field via the Ampère-Maxwell law; changing $B$ → electric field via Faraday's law), so the wave is self-sustaining and needs no medium.

Transverse nature: $\vec{E}$, $\vec{B}$, and the direction of propagation are mutually perpendicular; $E$ and $B$ oscillate in phase, with $E=cB$ at every instant.

Speed: $c=1/\sqrt{\mu_0\epsilon_0}\approx3\times10^8\ \text{m/s}$ in vacuum, matching the measured speed of light -- the discovery that showed light itself is an electromagnetic wave; also $c=\nu\lambda$.

Energy (qualitative): the wave carries real energy, at a rate (intensity, $\text{W/m}^2$) proportional to the SQUARE of its field amplitude; it also carries a small momentum, producing radiation pressure. …