1. No. Emigrating to another country is only the beginning towards getting citizenship of another country.
2. When you have satisfied the time requirement and other requirements (ie. language), then you will be eligible for citizenship of that country.
3. It depends. For example, Singapore and Germany does not allow dual nationalities and thus by acquiring another citizenship, that person will lose their original citizenship. On the other hand, China does not recogize dual nationalities so you can have another citizenship but will not be protected by Consular/diplomat of that country concerned.
4. Yes, except the above situations
5. Most countries allow dual nationalities but like I stated above, Singapore, Germany and Japan are exceptions to that rule. Australia, prior to 2004, also disallow dual nationalities but have since abandon that policy.
2007-07-24 21:39:31 補充:
To correct the above poster: US allow dual nationalities "The U.S. Government recognizes that dual nationality exists but does not encourage it as a matter of policy because of the problems it may cause. "
http://travel.state.gov/travel/cis_pa_tw/cis/cis_1753.html