SPECIAL

        In Japanese there are many special characters that do not necessarily make the sound they would do usually. As you know, a single kanji can have multiple pronunciations, depending on whether he is alone or not, for example. Similar phenomena occur with the kana, and especially with katakana. Since it is used for foreign words, katakana are required to be able to produce a wider variety of sounds that do not belong to standard Japanese. But these sound modifiers apply to the katakana too, and anyways some of them are required with the common Japanese language and I am sure you are already familiar with them. Here is the list of special kana:

• Long vowel and double consonant (the most common)

        The long vowel is placed after another kana and makes the ending sound of this one doubled:

hiragana:

う, u              e.g:          こう, kou (ko'o)

                                      ちう, chiu (chi'i)

katakana:

ー                  e.g:          ビー, biu (bi'i)

    Because it uses the "u", the effect of the "u" might restrain his usage because he cannot be preceded of anything without doubling its' vowel. But if it is absolutely necessary to put a "u" alone in the middle of a word, put a middle dot. It basically forces to make a small pause: 

E.g: チェック・イン (chekkuin)

kanji: (just repeats the entire preceding kanji)

々                  e.g:          団々, dandan

        The double consonant is in fact a small tsu placed in front of the desired character. Small kana are approximately 2/3 the size of normal kana and have very special properties.

hiragana:

っ   (compared to つ, tsu)     e.g:  もって, motte (mot'te)

                                                       たっし, tasshi (tash'shi)

katakana:

ッ  (compared to ツ, tsu)      e.g:  バッグ, baggu (bag'gu)

                                サンド-ウイッチ, sandouicchi (sandouitch'tchi)

• Special sounds
  

        Some sounds may not be created with the original Japanese language. Mostly Chinese, these foreign sounds are also made with the help of small kana (2/3 the normal size). Remember, I am not making this up! This stuff really exists! Even though some are extremely, but Extremely rare, they are sometimes necessary for foreign names. Don't forget to see the "special-weird" section. Here it is:

hiragana:

しゃ sha    ちゃ cha       Made with a shi (し) or chi (ち) and a  
しょ sho    ちょ cho       small ya(や) yo(よ) yu(ゆ), these associations
しゅ shu    ちゅ chu       allow the creation of sounds impossible                                                       with the normal kana. Take a quick look at                                                       the hiragana table, and you'll see.

katakana:

シャ sha     チャ cha      Same thing in Katakana.
シュ shu     チュ chu
ショ sho     チョ cho

        Some words require the skipping of a vowel, for example: Tôkyô, (the best example) has no vowel sound with the "k". Such sounds occur sometimes in normal Japanese; Kyo, Kyu, Ryo, Ryu. The others displayed here are pretty rare, but they still exist.

hiragana:

きゃ kya   りゃ rya     Ahh... the small kana are so useful! You can 
きゅ kyu   りゅ ryu     with the ki and ri with a small ya-yu-yo.
きょ kyo   りょ ryo     The same technique with Katakana.

    You can also put a ni or hi and do the same sounds (rare). Theorically, you can even use other "i" kana, but I personally never seen it. Examples:

にゅ nyu
びょ hyo

        Other sounds are impossible in the Japanese kana tables, as you might notice. Indeed, the ya-yu-yo column does not allow "yi" or "ye". Also, the wa column is forgetting about wi, wu, we and wo!!! Do not panic, these sounds are REALLY not useful and often have a close relative that basically makes the same sounds. For instance, yi is lot like "i" so, use your head! Wu is just like a simple "u", no? Wo is directly corresponding to the "o" that in is the table (Why is there two "o" characters? Good question, in fact, they have different functions as particles. Ok, for the other sounds, the clever Japanese found a way out of it... again with the small kana!

hiragana:

ふ        fu or hu       "f" sounds are made from this character
ふぁ    fa                 examples
ふょ    fyo

いぇ ye                  You can add small "e" or "i" to induce the sound into 
うぃ wi                  kana, but it is really rare. The "y" becomes a i and the
うぇ we                 "w" becomes a "u" kana.

ぢ di             Weird stuff.. Di and du actually exist!!
づ du

ぁ la              Obviously made for Chinese, these are made from small kana; 
ぃ li               a-i-u-e-o. When the "l" sound cannot be replaced by a "r"
ぅ lu              (which is close to the "l", remember?) these little kana are used.
ぇ le
ぉ lo

katakana:

These weird things are mostly seen in katakana since they are obviously for foreign terms. You can still write them with a computer adapted to Japanese characters.

        There are even more techniques to fool around with characters, but you'll be fine with what you got there. Remember that small kana automatically mean something special.