A question about offer

终于看见OFFER了， 是EMAIL给我的，我需要要求他们签字寄给我吗？

另外OFFER里有一句话看得我心里毛毛的：

It should be understood that this offer of employment is for at-will employment and is not an employment contract. Your employment and compensation can be terminated by the company or yourself with or without cause and with or without notice at any time.

请问这是通常OFFER里都有的吗？

多谢了！