Display additional field on dropdown select

Yes, very much possible.

For example, if you select 'Others' as an option in the select then a textbox needs to be shown where one can fill any description of other if needed.

You need to do:
1. Attach a Javascript 'onchange' handler for the select
2. Whenever anything is selected the 'onchange' will be triggered and you can check the selected value
3. If the selected value is the one that needs some extra steps then do it or else skip rest.
4. If it is the specific step, then via Javascript, access the other control (like a textbox), set its display property as true such that one can add more details.

Done! Try!