Some of the commonly used symbols:

I am aware of the ALT+= keyboard shortcut to get to equation editor mode. I also know of the CTRL+=, CTRL+SHIFT+= shortcuts to toggle superscript/subscript mode outside of equation editor. When in equation editor, those shortcuts do not work, and instead I have to click on the Script button from the ribbon.

infty - Infinity

leq - Less than or equal

geq - Greater than or equal

partial - Partial differential Adobe cs 5.5 mac os x yosemite compatibility 10.

sum - Summa

prod - Product Sign

subset - Contained in

in - Element of

cup - Union (if you want to see big symbol, enterbigcup)

neq - Not equal to

approx - Almost equal to (asymptotic to)

equiv - Identical to (equivalent)

int - Integral

oint - Contour integral

How to use all these symbols outside the equation, select the optionUse Math AutoCorrect rules outside of math regions in the Word Options. How to do it, seeChoosing Math AutoCorrect Options.

See also this tip in French:Raccourcis clavier pour entrer des symboles et des modèles dans équations.