Your question is too general to be answered accurately, but here goes:

1. You need to create a data connection to an email address to be able to
send a form via email.

2. The best way to do this is to create your own submit button (and not use
the default submit button of InfoPath), create a data connection to the web
service, and one to the email address, and use one rule with 2 actions to
submit the form to both the email address and the web service.

3. By using a web service that stores the data in the database and calling
that web service when submiting the form. Or you can bind your form to the
database and add a third action on your submit button to also submit the form
to the database.

4. The user needs to have InfoPath installed to be able to open the form (at
least for InfoPath 2003; this might not be necessary for InfoPath 2007, but I
haven't tried it out yet so cannot really say) as well as access to the
location where you published the InfoPath form template.

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S.Y.M. Wong-A-Ton