Often the words "Еще" [ishchyo] and "Пока" [paka] are seemed to have similar meanings but in fact they are very different. To understand this difference, you should know which parts of speech these two words are and what functions they accordingly perform.
Well, at first let's analyze the adverb "Еще":
1. "Еще" meaning an object, additional information.
Ребенок не слушается, да еще и отказывается есть. - The child doesn't obey and what's more - he refuses to eat.
Я сдал экзамен, да еще и победил в олимпиаде. - I passed my exam and won the academic competition.
2. "Еще" meaning recentness of an event.
Я родился еще в прошлом веке. - I was born in the previous century.
Он прочитал эту книгу еще в школе. - He read this book as far back as he was a school student.
3. "Еще" meaning "до сих пор" - by now/still.
Она еще не выучила это стихотворение. - She still haven't learned this poem.
Мы еще не были на Байкале. - We still haven't been at Baical.
4. "Еще" meaning additional opportunity.
Мы еще успеем на последний поезд, если поторопимся. - We still can catch the last train, if we hurry.
Он еще докажет, насколько умен. - He will prove how smart he is.
The word "Еще" can also be a particle. In this case it emphasizes the meaning of saying.
Он еще и спорит! - He also argues!
The adverb literally means "in the course of some time which lasts up to some event".
- Вы уже в театре? - Are you in the theatre?
- Нет, мы пока на работе. - No, we are at work yet.
You can use the word-combination "Пока не" [paka n'e] following the same principle:
Я пока учусь = Я пока не закончил университет. - I study yet = I haven't graduated university.
The conjunction "пока" means "for as long as".
Пока есть силы, нужно продолжать тренировку. - For as long as I have strength, I should go on with my training.
Пока мы на каникулах, есть время съездить на море. - For as long as we have our vacation, we have time to go to the sea.
The word "Пока" [Paka] is also a popular short of saying goodbye:
Пока, до скорого! - Goodbye, see you soon!