Electrons in atoms are located in regions of space surrounding the nucleus  known as orbitals. There are many types of orbitals, each differing in terms of their size, their shape and their spatial orientation. Electron configurations are a means of describing how many electrons within an atom are located in each of these orbitals.
 

Some Background on Orbitals
According to the Quantum Mechanical model of the atom, electrons are housed inside regions of space known as orbitals. There are many types of orbitals, each having a different size, energy level, shape, and orientation in space. The orbitals have names - like 1s, 2s, 2p, 3s, 3p, 3d, etc.  The number that precedes the letter indicates the energy level. The smaller numbers are lower in energy. So a 1s orbital is of lower energy than a 2s orbital. It is also a smaller orbital.

The s, p, d, and f orbitals have different shapes. The s orbitals are spherically-shaped. The p orbitals are shaped like pinched cylinders (cylinders pinched in the middle and bulging on their ends). The d orbitals are rather dorky shaped and the f orbits are shaped like frogs (... but if you really need to know their shape, don't take our word for it. Look at a picture instead. After all, not every person views all frogs to look the same.)

Each energy level has only one s orbital. Not every energy level has p orbitals, but those that do have three p orbitals. Not every energy level has d orbitals but those that do have five d orbitals. And finally, not every energy level has f orbitals but those that do have five f orbitals. Each orbital can hold a maximum of 2 electrons. Thus there can be up to two electrons in the s orbitals at any given energy level. There can be up to six electrons in p orbitals, as many as 10 electrons in d orbitals and 14 electrons in f orbitals. The table summarizes this information.
 

Orbital Type	Lowest E Level with This Type	# of Orbitals of This Type	Maximum # of Electrons
s	1st	1	2
p	2nd	3	6
d	3rd	5	10
f	4th	7	14

When writing an electron configuration, you list the orbital type, beginning with the lowest energy orbitals followed by a superscript to indicate the number of electrons in orbitals of that type. For instance, 1s² 2s² 2p³ indicates that there are 2 electrons in the 1s orbital, 2 electrons in the 2s orbital, and three electrons in the 2p orbitals. Further details about the process of writing complete electron configurations are shown in the graphic below.


Writing Electron Configurations for Ions of Elements
The graphic below presents the details of writing electron configurations.