The third alphabet, kanji, was imported over the centuries from China. The pictograms are typically (much) more complex than kana (remember, "kana" refers collectively to hiragana and katakana) and, furthermore, have different readings and meanings depending on how they're combined with other kanji. They represent ideas or words rather than syllables, although of course hiragana or katakana could be used to spell out the pronunciation.

It's not uncommon to see a kanji with its reading spelled out in very small kana, written just above it. Kana used in this way, to show you how to pronounce a kanji, are known as "furigana".

Also, the same kanji can often be read in different ways. For example, the kanji sei can be read as sei when in "sensei" (teacher), or as u in "umareta" (I was born), or nama in "nama biiru" (draft beer). Aargh! Panic! *slap!* Calm down. It's not that hard. Just breathe deeply and you'll be fine. Follow me...