What's the difference between `in pairs` and `in ipairs`, and how do I use them?

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-- Wiki

"Two commonly-used standard library iterator functions are pairs and ipairs. These functions return table keys and their corresponding values. pairs iterates through the entire table, even if the key is non-numerical (such as "Hi"), treating it as a dictionary. ipairs iterates through the table as if it were an array, starting at 1 and counting up consecutive integer indices. Once it hits a nil value, it will stop looping through the table. Also, please note that pairs does not iterate through the table in any particular order."