The T stands for Thumb instruction set which addresses the issue of code density. Specifically, Thumb mode allows instructions to be 16-bits instead of 32-bits thus reducing code density. A processor having the T suffix can thus run Thumb code.
The D stand for debug support. This means that the specific ARM7 you are using offers on-chip debug support, generally through a J-Tag interface.
The M means that the CPU contains a hardware multiply instruction.
I (EmbeddedICE macrocell)
Is the debug hardware built into the processor that allows breakpoints and watchpoints to be set.