what is difference between big-endian and little-endian?

see there: https://en.wikipedia.org/wiki/Endianness

to reverse the byte order of a 32/64-bit register is BSWAP instruction

Well, i want to develop a OS for Intel X86_64 architecture, what i need to notice about Endianness?

Perhaps the best thing to know is that the endianness is little for all x86 and all compatible CPUs. Least significant bytes at numerically smaller addresses.

Think i wrote a driver for USB 2.0, if i want to write memory to HDD, will i need to use BSWAP Instruction for converting memory to Big-Endian? Or if i want to read data from HDD, will i need to use BSWAP Instruction for converting data to Little-Endian?

And what is Offsets and Segment registers?

No, you don't need to use BSWAP unless you're reading something like a file header that uses big endian values.

A segment register selects a 64 kilobyte block of memory that you can then access with a general purpose register containing the offset into that block.

I admire your willingness to learn, but you should probably read some documentation and tutorials first instead of asking us a million questions.

Which tutorial? I find a Assembly tutorial on documentations, but it is was pretty old and so, writed for DOS.
Can you advice an up-to-date source?

If you're asking about segment registers, then 16-bit development in DOS would be appropriate. See the sticky post in the DOS forum: http://board.flatassembler.net/topic.php?t=9473

Unless you would rather target newer systems, in which case there are sticky posts in the other forums as well.