Difference between ji

I'm a student learning japanese and I'm wondering if there is any difference at all between じ and ぢ. I know they are both read as ''ji'', but I'm not sure if there is anything specific about each one. Along with that, what would I be more likely to see, or are they essentially interchageable?

Sorry if this sounds like such a pointless question.

I think there are rules in when to use each of じ and ぢ.

It's normally supposed to be じ (so this would be the one you'll see more often).
ぢ is used only on limited cases, which would be:
- Sound becoming voiced from unvoiced because of two consonants coming together
(e. g. ちぢむ = to shrink)
- Sound changed from unvoiced to voiced because two words were joined together, and the original word contained ち.
(e. g. はなぢ　= nose bleeding, from the words はな = nose, and ち = blood)

Apart from that, I would stay with じ.

- There is one word that uses ぢ in defiance of the above rules: the one-syllable word ぢ, meaning, um, hemorrhoid. I suppose this is because it is related to ち, ("blood").

- Other than the "official" rules, people intentionally use ぢ instead of the correct じ at times to make some words look humorous, such as writing おやぢ instead of おやじ (referring to "middle-aged men," or those "men that behave like middle aged men").

I found a good explanation on the web. Sorry in Japanese but see ;
detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1011024030

Briefly explained as follows.

The rule has set by Japanese MOE since 20 years.

(1) Principle
Use "ji" （じ）

(2) Exception of (1)
When two letters are put together, such as Hana (Nose) + Blood (chi) = Hanadi （はなぢ）

(3) Exception of (2)
But even two letters are put together, when people use ji customerily, rule (2) is NOT used. Example ; Sekai（世界：せかい）+ Chuu (中：ちゅう）= Sekaijuu(世界中:せかいじゅう）, chi （地：ち）+Men（面：めん）=Jimen（地面：じめん）

(4) Another Exception of (1)
When two sounds "si or chi" + "zi sound" continue, use the same charactors.
Example ;
Chidimu 縮む:ちぢむ
Tsuzuku:続く：つづく　

(5) Exception of (4)
Sorry but there is an exception of (4) !
Ichijirusii:著しい：いちじるしい

This is a general rule of ji(zi) and di. But except hanadi, you can think you should almost use ji.

...and the same goes with ず and づ. ;-)