보고싶다 >> It means "I miss you" 보고싶어? >> It means "Are you miss mi?" But if you use "보고싶어", it means "I miss you." I think you want to compare with "보고싶다" and "보고싶어". It is very similar so in normal case you can both of them freely. ("다" and "어" both of them are use very much. A little bit more "보고싶어" because "어" can use more friendly people or lover.) Both words can say to yonger / same age person. If you use this expression to elder person, you should use "보고 싶습니다." / "뵙고 싶습니다" Thank you.

This lesson could be helpful for you --- http://123learnkorean.wordpress.com/2009/08/26/%E3%84%B4%EB%8B%A4-%EB%8A%94%EB%8B%A4-%EB%8B%A4-plain-form/

They're pretty much 2 different ways of saying the same thing. It's just that the verb ending is different. I think 보고싶어 is slightly more informal. It can refer to things/events or people. But if you want to say "I miss you" to someone (such as a boyfriend), you use 보고싶어. If you say 보고싶다 to someone, it means something like "I want to see you".