Rendaku is a phonological phenomenon which affects Japanese compound words. When a native (Japanese) element which starts with a voiceless consonant appears as the second element of a compound word, the consonant becomes voiced.

Example: maki + sushi -> makizushi (this is of course Japanese food.)

In kana text, voicing is done by adding a voicing modifier (two little strokes) at the upper-right side of the character. Which, if you know hiragana or katakana, is an easy way to remmember that only the following consonants may undergo rendaku:

k -> g
s -> z
t -> d
h -> b

Note that rendaku only appears with Japanese elements. Compounds made of Chinese-Japanese elements, i.e the vast majority of compounds, do not undergo rendaku.

Furthermore, rendaku does not appear when the second element already contains a voiced consonant (Lyman's law). For example:

onna (woman) + kotoba (word) -> onnakotoba (feminine speach) instead of onnagotoba, because the "b" in kotoba is a voiced consonant.

Juuichiketajin gives an exemple of rendaku with a non-Japanese element:

irohagaruta = iroha + karuta, where karuta derives from the Portuguese carta.

karuta can be written with kanji, which indicates that the word is very well integrated in the Japanese language. That's probably why it is handled like Japanese words with regard to rendaku.