The I-94 arrival/departure record confirms that you have been admitted to the US. It's now an electronic document and is no longer issued in paper form (except in special circumstances).
You can download, save, and print a copy of your I-94 on the CBP website. Whenever you enter the US, you should save a copy of your I-94 for your records and verify that all information is accurate. If there are errors, let us know so we can help get them corrected.
Sample I-94 electronic admissions record