When dealing with web development you need to remember that there are two computers involved. The server that is hosting the web site and the user's computer. If you are wanting to execute code when a textbox's value changes, it just doesn't work the same as it would on a windows form. If it did, then every time a user presses a key in the textbox everything would stop while the data was sent to the server, the textchanged event code would run and then data would be sent back to the user and reload the entire page, etc. Not cool.
This is why you would likely want to do in javascript which runs on the user's computer and doesn't have to send data to/from the server. Basically, there is no textchanged event in javascript. You get
OnBlur[
^] which is basically a lose focus event.
Technically there is a TextChanged event for a web's textbox, but I the code only gets hit if a textbox value has changed between postbacks...and again, you don't want to do a postback every time a user presses a key in a textbox.
Hope this helps explain.