是 means 'is/am'. so for example: 我是男孩子 - I am a boy. 今天是星期四-today is Thursday. 就 emphasizes a fact. Example: when someone calls you, and they say '我找(your name)" (im looking for...) you'd respond "我就是“ which roughly means 'I am that person"(This is emphasizing a fact.) Another example: 他昨天就是睡觉和看电视- All he did yesterday was sleep and watch t.v. (here 就是 means 'merely/only').

The means of “就是” and “是” is similar ，but "就是“ is emphasis

不妨參考我的答覆 http://www.italki.com/answers/question/107411.htm

“是”表陈述，“就是”有强调的意思

是 = is 就是 = that's the one