What is joint custody and should i worry?

i have had physical custody of my daughter since she was born.she is now four.Her father just served me with papers wanting my daughter for just the summer,odd holiday,and requesting for joint legal custody..i live in idaho and he live in south carolina because he is in the millitary.im scared for her to go that far,but i agreed to it to start in 2013 when she is almost 7 as long as he comes and gets to know her during the summers and holidays.my questions are what is joint legal custody exactley?will he still pay chidsupport because ill have her most of the year?will him or i get the child tax credit?also if she is to scared to want to go with him or wants to come home can he force her?m just worried my baby will be scared since she has been with me this entire time and has only seen him maybe once a year.this all realy hard but i do want her to have her dad.im scared he will take her from me to at the same time.also can he request for me to pay childsupport for the summer?what can hedo with having joint and what can he not do?ive read alot of articles but just not getting the answeres im searching.i hope that this all dont sound crazy and to much im just stressing out.thank you