Your question is a bit vague.
What data is transfered by smartphones?
It depends. You can be fairly certain the data will include telephone conversations, SMS/MMS messages, and e-mails.
What data is stored on a smart phone?
- calls records (remote phone number, time/date of call start, call duration, direction [incoming/outgoing] , potentially the name of he remote party)
- SMS/MMS data ( photots [time/date, location, potentially names of people in the photos])
- location data ( time/date with lattitude longitude )
- photos ( see SMS/MMS )
- e-mails ( recipients, time/date, pop3 server, smtp server)
- web site usage ( URL, form data, cookies, time date of last visit )
- contacts (phone numbers, e-mail, physical address, relationships )
- music (preferences, listening frequency, etc)
Potentially, any data stored on the smartphone could be transfered.
Do you want any of this data to be private (confidential) ?
If so, then transfers involving this data need to be protected, usually this is done by encryption. The security risk in this case would be exposure of confidential data. Mitigation against exposure is encrypting your data, and configuring access controls to limit access to the data.
Do you want to protect any of this data from modification?
Protocols used during data transfer have the ability to modify or delete your data. The security risk in this case would be loss of integrity of the data (it gets changed), or loss of availabilty of the data (it gets deleted). Mitigation against loss of integrity is the use of cryptographic hashes (which will indicate if your data is intacted or not). Additional mitigation might be error-correcting codes which allow you to recover data that is altered. Mitigation against loss of availability is data backup and properly configured access control. The backup can restore the availability of data if it is destroyed, and the access control can limit who can delete which data.