There is no difference between "electromagnetic field" and "electric field and magnetic field". An electric field and a magnetic field are always just two components of an electromagnetic field, witch can be characterized by two vectors $\vec E$, $\vec B$. And electromagnetic field (= electric field + magnetic field) always obey the Maxwell equations.
However, there is indeed some context in which electric and magnetic fields are opposed to a single electromagnetic field. In the general case, when electromagnetic field changes rapidly enough, its electric and magnetic parts are strongly connected and it is impossible to consider them independently. And therefore they say that the electric and magnetic fields are two parts of a single electromagnetic field. But when the frequency of change of the field is small, then the connection between its two components — between the electric and magnetic fields — becomes weak, so that they can be considered as two independent fields.
The separation of the slowly changing electromagnetic field to (practically) independent electric and magnetic fields follows from the Maxwell equations.
Namely, if we take one pair of the Maxwell equations \begin{equation}\nabla\cdot\vec{E}= \rho / \epsilon_0 \\ \nabla\times\vec{E}= - \partial \vec B / \partial t \end{equation} and condition $\partial \vec B / \partial t \approx 0$, we get the following equations for an electric field alone: \begin{equation}\nabla\cdot\vec{E}= \rho / \epsilon_0 \\ \nabla\times\vec{E}=0 \end{equation}
Similarly, another Maxwell equations pair when $\partial \vec E / \partial t \approx 0$ becomes equations for a magnetic field alone.