Roots

In Indo-European languages such as English, the infinitive is usually the basic from of the verb of which the rest of the forms are derived.

For example, the infinitive "to talk" is the source of many derived words:

Talk                  Infinitive
Talking             Present participle
Talk                  Present simple
Talked              Past simple
Talk                  Noun

We see that the main stem of the infinitive stays preserved, while the inflection works by affixing other parts to the stem. At least it is so most of the time.

In Semitic languages things are a little bit more complex than that.
In Hebrew, the basic source of all the forms of a verb is called the "root" of the verb - שורש (shoresh)
The root is not a real word, rather it is a sequence of three consonants that can be found in all words that are related to it.

The consonants of the root are separated by different vowels in different words. They can also be separated by other extra consonants that do not belong to the root.
The root is used to make all the forms of a verb. It is used to make nouns as well.
Each root pertains to a certain meaning, e.g. K T B ك ت ب (Arabic), K T B כ ת ב (Hebrew) pertains to "writing".
See the following examples:
[note that when it comes to the roots of the verbs, the letters בּ (Beit [b]) and ב (Veit [v]) are considered the same letter. The same happens to כּ (Kaf [k]) and כ (Khaf [kh])] 


(he) wrote katab كَتَبَ
(he) wrote Katav כַּתָב

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writing kitaaba كِتابَة
writing ktiva כְּתִיבָה
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writen maktub مَكْتوب
writen katuv כַּתוּב

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booklet kutayyib كُتَيِّب
letter(mail) Mikhtav מִכְתָב