The Letters of the Hebrew Alphabet
Lesson 2.
Some Hebrew letters change pronunciation.
Historically there were six Hebrew consonants which in ancient Hebrew were pronounced with either a fricative [soft sound] or a plosive [hard sound.] These letters are known as BeGeD KaPHeT letters. [The name is simply an acronym for the letters 
: bet/
, gimmel/
, dalet/
, kaf/
, pey/
and
tav/
.]
The changes in pronunciation occur according to the position of the letter in the word and other factors, and are indicated by a small dot (called a dagesh.) Although at one time each of these six letters changed pronunciation, in modern Israeli usage only three change pronunciation—bet/vet, kaf/khaf, pey/fey. [The letter tav retains two pronunciations in Ashkenazi and Yemenite Hebrew.]
The Dagesh
The diacritical mark—the dagesh—is a small dot in the letter.
The dagesh has two forms—Dagesh Kal (meaning "weak/light", sometimes called Dagesh Lene) and Dagesh Hazak (meaning "strong", also called Dagesh Forte.) It is the Dagesh Kal that changes the pronunciation of the Beged Kafet letters.
Dagesh Hazak has a different function—it has the effect of doubling the consonant. As a general rule the dagesh, when found in the Bet, Kaf, and Pey in the beginning of a word or syllable, is Dagesh Kal. The Dagesh Hazak is indicated if its occurrence is preceded by a vowel. Dagesh Hazak is not limited to the Beged Kafet letters.
The Shuruk and Mapik
The Shuruk and Mapik may be confused with the dagesh. While they look the same each has a different function. The Shuruk is a small dot which follows a vav 
to form the vowel sound "oo." The Mapik is a small dot within the letter hey,
(and, rarely, an aleph) when it occurs at the end of a word. The Mapik indicates that the hey is pronounced.
The letter Shin
The Hebrew consonant Shin represents two slightly different sounds (phenomes). These sounds, sin /s/ and shin /∫/ (sh), are represented by the addition of dots to the letter Shin.
More about the Beged Kafet consonants...
Lesson 3. More about Hebrew letters.
Some consonants that take different forms
HERE.
