Vim defines word and WORD differently. For reference, here’s what :help word tells us.

A word consists of a sequence of letters, digits and underscores, or a
sequence of other non-blank characters, separated with white space (spaces,
tabs, <EOL>).  This can be changed with the 'iskeyword' option.  An empty line
is also considered to be a word.
A WORD consists of a sequence of non-blank characters, separated with white
space.  An empty line is also considered to be a WORD.

Let’s start with the simpler of the 2 - a WORD. A WORD is any collection of non-blank characters.

party                 #=> 1 WORD
Party!                #=> 1 WORD
Party! Party!         #=> 2 WORDs
Party!Party!Party!    #=> 1 WORD

Pressing W will move your cursor forward one WORD. If I am jumping ahead by word or WORD at any point, this is my preferred method. It’s easier for me to process whitespaces than it is to process whitespaces and parse text at the same time. And if I’m bouncing forward a few words or deleting the next x WORDS (which can be done with xdW) it’s likely because I’ve already counted the whitespaces.

The definition of a word is a little different. As :help tells us, it is still a sequence of non-blank characters but in this case there are some specific characters that actually separate each word. The “other non-blank characters” that make up a word are all listed and set in iskeyword. You can see these keywords by opening up vim.

:set iskeyword?
#=> iskeyword=@,48-57,_,192-255

Vim :help to the resuce. Details on the options formatting for iskeyword= are listed. Each option, separated by a ,, translates as follows.

@                 #=> all characters where isalpha() true
isalpha() == true #=> a-z, A-Z, plus accented characters
48-57             #=> ASCII codes 48-47, translates to number 0-9
_                 #=> Underscore. To include the at symbol use @-@.
192-255           #=> A unique collection of ASCII codes.

Everything else will separate a word. Building on our example from earlier:

party                 #=> 1 WORD, 1 word
Party!                #=> 1 WORD, 2 words
Party! Party!         #=> 2 WORDs, 4 words
Party!Party!Party!    #=> 1 WORD, 6 words
Party, party, party!  #=> 3 WORDs, 6 words
party.count           #=> 1 WORD, 3 words

You can navigate through each word with a w. Phrases like party.count are both very common and a big reason I don’t like to bother with w as often. If I have a line like party.count if > party.start_time deleting a few words feels strange to me when I’m counting ..

Vim is all about manipulating and navigating text objects. Deep dives like this are a great way to help understand exactly what is happening each time you press a key in vim.