Your NetID is made up of random letters and numbers (there may also be an underscore, ex: d_b123, or db123), and you use it to access services like Canvas, Catsweb, or library databases. This would also be your TXST email when adding txstate.edu to the end: d_b123@txstate.edu, or db123@txstate.edu.
You use your Student ID number to access various Texas State services such as class registration and financial aid.