What is the "Grave" button?

This is usually the button immediately above the TAB on most keyboards.

For my keyboard - grave is actually correctly known as the ` key - grave is also known as the back-tick.

In my travels I have seen the Ubuntu "key above the tab" to actually be the § or º. I'm sure there are many other similar characters on peoples keyboards.

Depending on your locality if you press and hold the SUPER to display the overlay, you should see the equivalent Grave key for your locality

for example:

_{(thanks Javier Rivera)}

For my keyboard combination - ALT+` - gives you this switching capability (cycle through the windows group):

To use this key combination - let go of the grave without letting go of Alt and then repress grave to cycle through the windows group.

Additional information:
List of most common Accents and Diacritical marks

acute       ´  
breve       ˘  
caron       ˇ  
cedilla     ¸  
circumflex  ^  
dieresis    ¨  
grave       `  
macron      ¯  
ogonek      ˛  
tilde       ˜  

EDIT:

tilde | ˈtildə | noun
an accent ˜ placed over Spanish n when pronounced ny (as in señor) or Portuguese a or o when nasalized (as in São Paulo), or over a vowel in phonetic transcription, indicating nasalization.

• the same symbol as a part of a URL.
• a similar symbol used in mathematics to indicate similarity, and in logic to indicate negation.

ORIGIN mid 19th cent.: from Spanish, based on Latin titulus.

V.S

macron | ˈmāˌkrän; **ˈmak-; ˈmākrən | noun
a written or printed mark ¯ used to indicate a long vowel in some languages and phonetic transcription systems, or a stressed vowel in verse.

ORIGIN mid 19th cent.: from Greek makron, neuter of makros ‘long.’

Source:

Good question yossile (+1)up.
First of all, both differentiate in visual appearance ¯ vs. ˜. Secondly, they are not exactly the same, depending on the language they where adopted by as shown in the dictionary example above, though both share a lot of similarities in executed usage, due to there common origin.

To my knowledge, the dictionary quote above is "not accurate/adequate", since one can find ˜ in archaic Greek philology (Heraclitus - Logos, Empedocles, Parmenides etc..) as well. Although the Greeks adopted it from the Phoenicians Alphabet, which our language system today mainly is build on (Alphabet/Letters).

"Grave" button is `, just next to 1.

On German keyboard it's the Ö button.