An inductor consists of a coil of wire, as its symbol shows:

The image above shows two coils. Inductor L1 has no core; the wire of L2 is wrapped around a ferrite (soft iron) core.

What will happen if we connect a DC voltage source (battery) to an inductor?

As a child, you may have built an electromagnet by wrapping copper wire around a nail and connecting it to a battery. Without knowing it, you actually created an inductor. Now you know what happens if you connect an inductor to a battery: you end up with an electromagnet. Be careful though; small coils you buy in a store are made of very thin wire. If you connect it to a battery, the high current may overheat the inductor within a second.

Although electromagnets are very useful, things get really interesting if we connect an inductor to an AC voltage source. When you disconnect an inductor from its battery, it wants to keep its magnetic field. Reversing the magnetic field is therefore not an easy task. But that's exectly what an AC current wants to do: it wants to reverse the magnetic field over and over again. The higher the frequency, the harder this will be. In other words: an inductor blocks AC currents, but conducts DC currents.

The unit of inductance is Henry, symbol H. In schematics, you usually find mH (miliHenry) and uH (microHenry).