Both DSEG ( for Data Segment ) and CSEG ( for Code Segment) are assembler directives and the assembler manual should clear them up for you as has already been mentioned. The assembler uses these directives while creating the object file from the assembly file.

And for further understanding you should be able to differentiate between the Internal RAM / Indirect RAM / Bit Mapped RAM / SFR area / External Data RAM / External Code RAM / Internal Code RAM. All these areas will be covered by the bible - maybe a few repeated readings are required ( and switching off the TV in front of you normally helps grasp faster !!)

Raghu