What is "_(letter)," and whats the difference between pair and pairs?

1) _ is treated by Lua as a letter: exactly the same as A or e or F. For stylistic purposes, it's sometimes used as a word or namespace separator: Ex.: apple_pie sourceEngine_lightFires

When it precedes a name, it usually indicates that the name is somehow special or not intended to be accessed. For instance, a helper function for a factorial may be called _factorial, and the function you are supposed to call is called factorial. Other examples are the metamethods, which are all preceded by two underscores: Ex.: __index

Finally, sometimes we use a single underscore to indicate that a variable is not important. Because of how multiple returns work in Lua, this is sometimes necessary.

For example, string.find locates the first and last characters of a found string. So if we only cared about the last character, we might see someone doing something like this:

_,last = str:find("cat");
-- last is position of "t" in "cat" in `str`
-- we don't care about _ so we called it that to indicate that

All of this said, those are just common stylistic considerations. There is nothing special about the underscore.

All of the following are valid, normal names that act like any other function or variable name:

_under_SCORE
_sdofij_WOEIJFOIJ___SDOIFJOSDIJ
_sdoifj_____
______
_2_3_4_
___
_
_______

2) pairs and ipairs both iterate over tables. pairs iterates over all keys/properties/indices, while ipairs only iterates over those which are counted in table length (#), which are the values 1, 2, 3,... until a value is found which is nil.

This means that only "a","b","c" will be printed out for the following, even though the positive integer key 5 is defined for the table:

local t = {"a","b","c","d","e"};
t[4] = nil;
for i,v in ipairs(t) do
	print("t[",i,"] = ",v);
end