I need to print out a small tag of 10cm x 4cm (w x h) but I'm not sure what to put in the resolution. I set the width and height to 'centimeters' as measurement but not sure what to put as resolution. Is it 300 pixes/inch or 300 pixels/centimeter (since I used centimeters as a unit)?
The unit is 300 ppi. So if you convert it to cm you need to divide 300/2.54=118.11 ppcm.
Use 118 ppcm or 300 ppi.
But try not to use Photo-IUsethisProgramForAllButIShouldNot-Shop, but a vector based one, like Ilustrator, Corel, Inkscape, Scribus, Indesign, Serif Page Plus.
As Rafael points out, 300dpi is the same as 118dpc (dots per centimeter?)
Note that 300dpi is 'standard' for printing photographs. If your Artwork is text or line-art based, however, you want to be printing at much higher resolutions. 600dpi at minimum (if not higher at 1200dpi for high quality printing).
Just to let you know, ppi is used for web, and dpi for print.
For exporting the regular values are 72 ppi for normal, 325 ppi if you have retina displays in mind, and 300 ppi for print, some printers can take higher, but normal ones 300 is enough.