It's a big term... According to this page >>> http://dict.baidu.com/s?wd=%B0%C9&f=3 and (which is in the '详细解释' box) >>> http://www.zdic.net/zd/zi/ZdicE5Zdic90ZdicA7.htm Except the usage as an 'onomatopoeic' word (word to imitate some sound) and the transliteration of the English word 'bar', roughly explaining as follow (when used as an auxiliary word) 1. (used at the end of a sentence) indicating entreaty, suggestion, command, etc 2. indicating agreement or acknowledgment 3. indicating doubt 4. indicating uncertainty 5. indicating a pause

when you ask someone to do something together with you, you can use it.

ba is a modal particle.it use for exhance your mood.like 这样不好吧。(it is not ok.) 吧是个语气词，它是用来加强你的语气的。

it is used when you say something not sure, or suggestion. exp.: Please take it. (we can say 请带上它 or 带上它吧。 请 sounds formal , while 吧 sounds familiar.) Maybe.(可能是吧。 吧 here means you are not sure)

It is a Modal particles,You can use it in lots of case and express different means! Sometimes express "have to do " or "happy to do"!