Why do odds move?
Odds move because the book's risk changes or the information does. Money moves a line, most of all money from bettors the book respects. So do injuries, weather and what the other books are dealing.
Money
A line is a price, and like any price it reacts to who is buying. When one side takes a lot more money than the other, the book is exposed on that side, and moving the number makes that side less attractive and the other side more so.
Not all money counts the same. A book knows its players, and one bet from a sharp it respects can move a line while $20,000 of public money on the other side moves nothing. The book isn't reacting to the size of the bet. It is reacting to who made it.
News
Some moves have nothing to do with bets. A quarterback is ruled out, a starting pitcher is scratched, the wind picks up and a total drops, a lineup comes out and a star is resting. The book takes the game down, reprices it and puts it back up. If you see a line jump several points at once, I'd look for news before looking for a conspiracy.
The other books
Books watch each other. A handful of them put up numbers early, take limited bets from sharp players, and let that action shape the line, and most of the rest copy the result and follow it all week.
A book that is out of line with everyone else gets picked off: players bet one side with it and the other side somewhere else, and collect on the difference. So a book will move a number to match the market without having taken a bet on it.
The balanced book is a myth
You will hear that a book wants exactly half the money on each side so it can collect the vig and go home. It is tidier than real life.
Books take positions all the time, most often against the public, and they move a line when the risk is more than they want to carry, not every time the split is uneven. A book can be very lopsided on a Sunday night game and perfectly happy about it. I'd rather be lopsided against the public than balanced any day.
When to bet
Think about when you bet, not only what you bet. If I liked a popular favorite, I'd bet it early, because the public will arrive later and the price will get worse. If I liked an underdog the public won't touch, I'd often wait and get a better number. And keep a record of the line you bet and the line at kickoff. If you regularly get a better number than the close, you are betting well, whatever last week's results say.
The number you bet matters more than the team you bet.
Related questions
What is the vig?
The vig is the bookmaker's cut, built into the price. At the standard -110 you lay $110 to win $100, and that extra $10 is how the book gets paid.
What does minus 110 mean?
Minus 110 means you risk $110 to win $100. It is the standard price on a point spread or a total, and the extra $10 is the book's cut.
How do betting odds work?
Betting odds are a price. They tell you what a bet pays and, read the other way, how often it has to win. Odds of -200 mean you risk $200 to win $100. Odds of +150 mean you risk $100 to win $150.