The best one-sentence description of inflation I've heard is: inflation is a hidden tax on savings. People who save money are hurt the most. It's as if the newly printed money is generated by taking a bit from anyone according to how much cash he/she has stored away.
You could have scenarios where the central bank does not change policy but money supply increases because banks are lending more money and therefore using the discount window more.
Also people could expect more inflation in the future, so they try to get rid of cash faster to buy goods. This increase in money velocity will also increase inflation.