1) That's only one possible source of taxable income. For an EXTREMELY rough list, look at lines 7 through 22 of the 1040.
2) Yes.
3) taxable income is the money you are paid from any and all sources, minus allowed deductions. For example, in 2018 you will get a $12K standard deduction. If filing MFJ then it's $24K. So the first 12/24K of your gross income will be tax exempt for the sole reason that you breath. Then if you pay qualified mortgage interest on your house, that interest you paid in 2018 is deducted from your taxable income. Then there's tens of thousands of other possible deductions, of which all won't apply to you of course.
Basically, your taxable income is on line 43 of the 1040. That line is labeled "Taxable Income". On the 1040-A it's line 27.
