1) In TCP, does checksum is done also over the data field?
In http://www.ietf.org/rfc/rfc793.txt site, it is said that "The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header and text"
2) what do they mean by the term "text"?
The "text" referenced is the body of the packet. So yes, the checksum includes the data.