Cyrillic is the script of many Eastern European Orthodox nations, most famously the Russians. Cyrillic is similar to Latin and some letters even look the same (but there are more of them).
Some 250 million people (3,8% world population) use Cyrillic but its usage zone covers 14,22% total world land area.
Cyrillic letters
Every cyrillic letter means a single sound (a consonant or a vowel).
Every letter has two forms: capital and non-capital. As in Latin script capital form is used only in some circumstances (first letter of sentences, names, etc.) and all the other letters are non-capital. Non-capitals are usually smaller versions of capitals.
The most popular version of Cyrillic is today the Russian one. Balkan languages have some additional letters that are no longer used in Russian. Most post-Soviet languages that use Cyrillic have all the Russian letters and some additional ones with diacritic marks.
History of Cyrillic script
By 10th century AD Europe was divided into Catholic (Western) and Orthodox (Eastern) spheres of influence. Writing was directly related to religion at the time as most literature was religious. Catholics prefered Latin language and script while Orthodox prefered Greek.
By 10th century however first Slavic nations (Serbians, Bulgarians) adopted Orthodox faith. Greek script had too few letters for the sound of their Slavic church language. After multiple attempts to devise a new script Cyrillic, created by saints Cyrill and Metodhius, prevailed.
Cyrillic would have remained a local script of Blakan Slavs if not for Russia adopting Orthodox faith (and thus Cyrillic script). Russian Empire expanded at unprecedented levels in the next centuries, acquiring North Asia (Siberia), Caucasus until the 19th century. Russian settlers settled these far-away lands.
Cyrillic script was also adapted to some previously unwritten non-Slavic local languages of the Russian Empire. More controversial were attempts to force some minorities that used Latin or Arabic script to adopt Cyrillic. These started in the 19th century (Lithuanian, Belarusian languages) but gained a full swing in the 1930s after the communist revolution established Soviet Union. Almost every nation of the Union has been forced then to adopt Cyrillic. During the Cold War (1945-1990) Cyrillic thus became a symbol of the communist world.
After the Soviet Union collapsed in 1991 some newly-independent nations abandoned Cyryllic for Latin (Azerbaijan, Uzbekistan, Turkmenistan). However the nations that remained in the Russian Federation have been barred by law from abandoning Cyrillic.