PS: 
BA:  The speaker ends with a BA to express his or her hope for the affirmative answers from the listener sometime. 

MA: not sure 
BA: affirmative. e.g. -she's not a pretty girl BA! or let's go kill some pretty girl BA!

hello! when you find it in the end of a sentence,sometimes it means the speaker is not sure about his/her opinion which you dont need to answer. it can also appear in the end of suggestion.

“吧”used at the end of a sentence（） ：
1.Indicating entreaty, suggestion, command, etc,For example: Do you agree that we go to amusement park? I suggest to go swimming.You ask everybody's opinion(我们去游乐园吧！去游泳吧！你问问大家的意见吧！）
2.Indicating agreement or acknowledgment.For example: Ok, let's go swimming(好吧,我们去游泳)
3.Indicating doubt.For example: Everyone agreed to go swimming?(每个人都同意去游泳了吧？)
4.Indicating uncertainty or speculate.For example:Should be agreed!(应该都同意了吧！)
5.Indicating a pause.For example:If someone does not agree,go, bad,don't go,bad also(如果有人不同意，去吧，不好，不去吧，也不好)
“吧”Can also be an onomatopoeic For example: Handgun voice:papa（手枪的声音：吧吧）
“吧”Also can be the noun For example: wine bar(酒吧)
Expression is bad, the hope can help to you<img alt="" src="http://www.italki.com/fckeditor/editor/images/smiley/msn/regular_smile.gif" />

They all means you are asking a yes or no question.
The difference is:
1.你会了吧？ ni hui le ba?  You understand?
2.你会了吗？ ni hui le ma?  Do you understand?