What is exactly is a bit or byte?

Answer 1

Seriously?
Bit is the smallest information unit, containing either a zero or a one. 8 bit = 1 byte.

Answer 2

Bit - binary digit 0 or 1
nibble 4 bits 0011
1 byte = 8 bits 0011 1000
word - 32 bits or 64 bits depending on your computer
Dword 2 words
Quad 4 words
etc.

Answer 3

bit and byte are units like kilogram or millemeter but they are information units!
bit is the smallest unit and byte = 8 * bit
kilobyte = 1024 byte .. and so on