Various encodings of mov instruction

Question

I have the following assembly code for moving immediates into an 8,16,32,64-bit register:

.globl _start
_start:
    mov $1, %rax
    mov $2, %eax
    mov $3, %ax
    mov $4, %ah
    mov $5, %al

And when debugging in gdb:

 0x0000000000401000  48 c7 c0 01 00 00 00  ? mov    $0x1,%rax
 0x0000000000401007  b8 02 00 00 00        ? mov    $0x2,%eax
 0x000000000040100c  66 b8 03 00           ? mov    $0x3,%ax
 0x0000000000401010  b4 04                 ? mov    $0x4,%ah
 0x0000000000401012  b0 05                 ? mov    $0x5,%al

And on the intel page it shows:

Some questions on the Opcode column:

• What does the +rb or +rw or +rd mean?
• I'm guessing the ib, iw, id, means immediate-byte, -word, -double- is that correct? What does the io in the last row mean?
• What is the REX prefix on the ah instruction that increments the op code from b0 to b4 ?
• Why does the mov imm, r16 require the 66 prefix? It doesn't list any prefix in the row for imm16. It seems the 66 just flags this at a 16bit register?
• Finally, why the three-byte code, 48 c7 c0 for mov imm, r64? I don't see