A very interesting characteristic of the Arabic language is the root system. Usually, each word is composed of three (though sometimes two or four) central consonants. These consonants are referred to as the جذر (pronounced jathr with the “th” sound like in “this”), or “root”. Each جذر has a general meaning and from this other similar words in categories like verbs, nouns or adjectives can be derived. In order to derive these other words you must know the pattern (وزن / wazn) into which you should put the root. There are certain patterns for past tense verbs, present tense verbs, gerunds, nouns of location, etc. into which you can place the consonants of the root.
An example of the root system is the root: ك ت ب. This combination of letters means the derived words have to do with books and writing.
By placing these three letters into various patterns you can derive the following words as well as many others:
مكتب / مكاتب offices/office
كتب / يكتب to write (present/past tense)
مكتبة / مكتبات libraries/library
كتاب / كتب books/book
Knowing the root of a word and its meaning is important for two reasons. First, it is possible to derive the meaning of unknown words from the combination of the root and the pattern or at least get a general meaning of the unknown word(s). Secondly, most Arabic dictionaries are organized by the root system. You have to be able to extract the root from the word by knowing the pattern. Then, you search for the word under the three letters of the root rather than the first letter of the word as they are generally listed in English dictionaries.