That Vsauce video really has done some damage, huh
The smallest infinity is the countable infinity. It is the cardinality (think 'size') of the natural numbers (1,2,3,4,...), hence the name.
Unintuitively, the whole numbers (Natural numbers, 0, and Negatives) have the same cardinality. That means you can match up each natural number with a whole number one-to-one. ('there exists a bijective function')
Even stranger, the rationals (-½,1.3,16.6...) also have the same cardinality as the naturals. The proof is a bit more involved, but still not that hard.
Now, what infinity is larger than others, then? This is where we find the Reals (non-terminating decimals, π, e, √2). No matter what you do, you cannot match them up with the naturals. If you're curious about that, look up Cantor's diagonal argument.
But, interestingly enough, the numbers between 0 and 1 have the same cardinality as the Reals! Any interval within the Reals is the same 'size' of infinity as the entire Reals. You can always find a one-to-one correspondence between the two. (For (0,1) and R you could pick tan, for example)
More generally, if you want to produce a 'larger' cardinality from an existing infinite set, you can look at it's power set. That's the set that contains all possible subsets from the original, and always has a larger cardinality than the old one.
tree() is a different function to TREE()