The difference between ^ and [^] is that in the first case the caret ^ is a meta-character that matches the beginning of the line
(it's a zero-length match, it matches the empty space at the beginning of the string, not the first character of the string).

The $ character matches the end of the line/string.

They are called anchors.

In the second case the brackets [] denote a character class. If the first character of the list is the caret ^ then it matches any character not in the list,
so [^ ] matches any character different than a space (you have a space after the ^ character).

In this case we have:

Code:

 ^\([^ ]*\)

The first group of parentheses () saves any number of occurrences (because of the * after the complemented character class [^ ])
of non-space characters [^ ]. As far as the following string is concerned:

Code:

CPU Util SSUUUUU | 16% 9% 22%

It's the following string:

Code:

CPU

Followed by:

Code:

 [^0-9]*\([0-9]*%\).*$

A space, any number of occurrences of the following complemented character class [^0-9], i.e., any non-digit character. In this case the string:

Code:

 Util SSUUUUU |

Then we save the string matching the following regular expression:

Code:

\([0-9]*%\).*$

any number of occurrences (*) of the indicated character class [0-9] - only digits, followed by the % character,
followed by any number of occurrences (*) of any character . (dot).

It's the following string:

Code:

16%

You can refer to the previously saved matches using the spacial syntax \1, \2 and so on.

Hope this helps.