To see how the capitalization of words works, go to: Capitalization
In computing, a word is an integral data type, that is usually (but not always) equal to the size of addresses used by the CPU to access the RAM (that is, pointers). Due to the popularity of the early Intel x86 processors (which were put inside IBM PCs), a word came to mean 2 bytes (16 bits), even though the Intel 80386 and higher use 4 byte addresses (see x86).
In the Christian religion, the Word of God is the second person of the Holy Trinity, Jesus Christ. Word, in this sense, is a translation of the Greek logos which more broadly means reason, principle, standard, logic. Logos also has parallels to the eastern concepts of Tao and dharma.
In urban slang, "word" is an exclamation indicating deep and complete agreement.