When you convert qualified leads, Salesforce moves any campaign members to the new contacts. The leads become read-only records, unless your administrator gives you permission to edit them. If existing accounts and contacts share the same names as those specified on the leads, you can choose to update the existing accounts and contacts. Salesforce adds information from the lead into empty fields; Salesforce does not overwrite existing account and contact data.
1) https://help.salesforce.com/apex/HTViewHelpDoc?id=leads_notes.htm

What happens when I convert leads?
https://help.salesforce.com/apex/HTViewHelpDoc?id=faq_leads_what_happens_when.htm

http://salesforce.stackexchange.com/questions/42212/what-happens-when-a-lead-is-converted

When you convert leads, Salesforce creates new accounts, contacts, and, opportunities using information from the leads you’re converting. Salesforce moves any campaign members to the new contacts, and the leads become read-only records. If existing accounts and contacts share the same names as those specified on the leads, you can choose to update the existing accounts and contacts. Salesforce adds information from the lead into empty fields; Salesforce does not overwrite existing account and contact data.

All open and closed activities from the leads are attached to the accounts, contacts, and opportunities. You can assign the owner of the records, and schedule follow-up tasks. When you assign new owners, only the open activities are assigned to the new owner. If you have custom lead fields, that information can be inserted into custom account, contact, or opportunity fields. You can’t view converted leads, but they appear in lead reports. Salesforce updates the Last Modified Date and Last Modified Bysystem fields on converted leads when picklist values included on converted leads are changed.
https://certifiedondemand.com/configuring-salesforce-lead-conversion/

Lead Conversion Mapping
1)https://help.salesforce.com/apex/HTViewHelpDoc?id=lead_conversion_mapping.htm&language=en

Let us know if this will help you

Thanks
Amit Chaudhary