256 MBit technology refers to the manufacturing process.
Unless you are a semiconductor engineer, there really is no need to explain beyond that.
You can find all kinds of explanations, though.
If you join JEDEC, you may find many helpful documents regarding the standards of the production processes.
Other info is proprietary, and available from the likes of Micron, Hitachi and Samsung.
What you need to know (as a user/purchaser) about the RAM is this:
The RAM expansion slots accept 168-pin SDRAM DIMMs that are 3.3 volt, unbuffered, 8-byte, nonparity, and PC133 compliant.
DIMMs with any of the following features are not supported in the Power Mac G4: registers or buffers, PLLs, ECC, parity, or EDO RAM.
If you are a developer, you need to know this:
The mechanical design of the SDRAM DIMM is defined by the JEDEC MO-161-D specification.
The electrical design of the SDRAM DIMM is defined by the JEDEC standard 21-C specification.>The presence detect serial EEPROM specified in the JEDEC standard is required and must be set to properly define the DIMM configuration. Details about the required values for each byte on presence detect EEPROM can be found in sections 4.5.4 and 4.1.2.5 of the JEDEC standard 21-C specification.
Capacitance of the data lines must be kept to a minimum. Individual DRAM devices should have a pin capacitance of not more than 5 pF on each data pin.
If you are trying to figure out acceptible density of individual SDRAM chips on the DIMM, the following table will help:
RAM Addressing
So, as you can see, following the advice given by others, and buying only quality Guaranteed Mac Compatible RAM from trusted Mac specialty suppliers is the easiest way to guarantee success.