The letters are only half of reading Arabic. Small marks above and below decide what each letter actually says — the same bāʾ can be ba, bi, bu, or no vowel at all. Every example below uses the same letter, so the mark is the only thing changing.
Fatha الفَتْحة
A short stroke above the letter. It adds a short "a" after it.
In a word
بَلَد
balad
town
Kasra الكَسْرة
The same stroke, but below the letter. It adds a short "i".
In a word
بِنْت
bint
girl
Damma الضَّمّة
A small wāw above the letter. It adds a short "u".
In a word
بُرْج
burj
tower
Sukūn السُّكون
A small circle above the letter meaning "no vowel": the letter closes the syllable instead of opening one. That is why it is shown here after a vowel — a sukūn can never begin a word.
In a word
قَبْل
qabl
before
Shadda الشَّدّة
A small w-shape above the letter, doubling it. You hold the letter about twice as long. It always sits with a vowel mark of its own.
In a word
رَبّ
rabb
lord
Tanwīn التَّنوين
The vowel mark written twice, adding an "n" sound at the end. It marks an indefinite noun — the difference between "the book" and "a book". Each of the three vowels has its own form.
In a word
كِتَابٌ
kitābun
a book