Arabic words are either masculine or feminine. For example: رجل (ra-jul) which means man is masculine. امرأة (im-ra-ah) which means woman is feminine. These two examples are very logical. But just like many other languages, everyday objects have their own gender as well. For example the word for ear, must be either masculine or feminine.
The most common way to tell a masculine word from a feminine word is the ة (taa' mar-boo-Tah). If a word ends in a ة (h/t) then it is most likely feminine. To make a masculine word feminine usually a ة (h/t) is added to the end of the word.
For example:

كلب

kalb
kalb
dog

كلبة

kalba/kalbah
kal-ba
female dog, she-dog

In Arabic nouns, as well as adjectives, have to agree in gender, as well as number.
For example:
الولد كبـيـر

al_waladu kabeer(un)
al-wa-la-du-ka-beer
The boy is big

البنت كبيـرة

al_bint kabeerah(-tun)
al-bint-ka-bee-rah
The girl is big.

Did you notice that we called the girl "big" with كبيرة (ka-bee-rah) while when we want to say the boy is big we said كبير(ka-beer). This is generally how it works for singular nouns.

الولد صغير

al_waladu Sagheer(un)
al-wa-la-du-Sa-gheer
The boy is small

البنت صغيرة

al_bintu Sagheerah(-tun)
al-bin-tu-Sa-ghee-rah
The girl is small.

الولد قصير والبنت طويلة

al_waladu qaSeer wa al_bintu Taweelah(-tun)
al-wa-la-du—qa-Seer—wal-bin-tu—Ta-wee-lah
The boy is short, and the girl is tall.