I've answered another similar question before.

See: http://nboard.nciku.cn/qna/question/10000000000000033066

Hope it can help you.

To some extent, they are interchangeable. Both of them mean "the same","similar", but "一样" individually has some other meanings.

And "同样" is often used as a adv, while "一样" is often used as a adj.

For example:

1.同样

Similarly, machines could also be used to keep a check on a patient's health record and bring it up to date.

同样，机器能够检查病员的病因，并使之反应出最新情况。  

Here, "同样" is a adv.

2.一样

(1) the same

The machine developed the same fault again.

这台机器有出现了一样（同样）的毛病。

It is boring to listen to the same story.

听同样（一样）的故事是令人厌烦的。

Here, 同样 and 一样 are interchangable, both of them are used as adj.

(2) like

He moved like a tiger stalking its prey.

他像老虎一样悄悄潜近他的猎物。

She is gentle like a lamb.

她像只小羊羔一样温柔。

(3)an (object, item or article)

She gives me a talisman.

她给了我一样法宝。