Dago answered #1.
Number two was is an old term from science and engineering. It was a more accurate SWAG. You and your engineering buddies would set down at the local dive and pencil out the performance parameters of that new fighter you were all dreaming of or that new big block muscle car on the back of the a handy piece of paper. It's also called a back of the napkin calc.
Three is a different version of SWAG. It means that you feel that the number your guessing will be within a factor of ten of the actual amount. Astronomers and people that have to estimate very large numbers use it. Example: I think that there are 1000 widgets out there. An order of magnitude would be 100-1000 widgets.
Four is what we do everyday on the computer. In the old days computer time was very expensive. You had to pay for every second you were using the mainframe. There were really no other computers but main frames way back when. Everyone had to fight for time on these computers. So you would log in when you started and log out when you were done. It meant that back then you real tried extra hard to optimize your code and do your best to make sure it was right the first time. They still do it today for real important computers.
