IOSTANDARD is an optional constraint that selects the desired I/O signal standard. LVCMOS25 means 2.5 volt CMOS. See the Spartan-3 data sheet, Table 2, "Signal Standards Supported by the Spartan-3 Family".
Each I/O buffer provides additional options such as slew rate and drive strength . Those two are very helpful for fine-tuning signal quality when driving long PCB traces. The various options are described in the data sheet, user guide, and ISE Constraints Guide. They are easy to put into the UCF file, like this:
NET "TXEN" LOC="B17" | IOSTANDARD=LVCMOS18 | SLEW=FAST | DRIVE=4 | PULLDOWN;