When entering the U.S. by air, the traveler will need to obtain her I-94 record by going to www.cbp.gov/I94. At land ports of entry, I-94 cards are typically not issued. It is very important to check for your I-94 record online to verify your period of admission.
If you are given a 6 month I-94 record, and you depart the US and seek readmission while the initial I-94 remains valid, you will only be admitted for the remaining time on the I-94; you are not given a new 6 month period.
Always check your I-94 record after each entry to make sure you do not overstay your period of authorized stay in the U.S.!