"Is there a difference between volume and decibel level?"

Volume is not a well-defined measure of sound.  Decibel is defined...as a ratio.

If you have two audio signals, and one is a ten times stronger than the other, you can say it is 10dB stronger. If it is 100x stronger it is 20dB, if it is 100x weaker, it is -20dB. It's a logarithmic formula.

Sound pressure levels (SPL) given in dB refer to 20microPascal at 0dB, which is actually around the threshold level of hearing anything at all.

Here is where that ratio comes into play.  I'll use an everyday example.  Let's say you are listening to your headphones in a completely quite environment.  Let's say in that environment, the dB level of the headphones is 80.  With headphones still on, you walk outside and start your gas-powered lawn mower.  You find yourself increasing the volume on the headphones.  Yet, when measured in that environment, the headphones are at 80dB again.  What's up with that?

As decibel is a ratio, it's based on some baseline level.  In the first instance, absolute silence.  In the next instance, the sound of a lawnmower.  Therefore, in the second instance, your ears are hearing the decibel level of the lawnmower + the decibel level of the headphones.  You could be sending in 130dB into your ears.  According to OSHA, that's within pain range.

Here is a chart from OSHA that might help:

 TABLE G-16 - PERMISSIBLE NOISE EXPOSURES (1)
______________________________________________________________
 |
 Duration per day, hours | Sound level dBA slow response
____________________________|_________________________________
 |
8...........................| 90
6...........................| 92
4...........................| 95
3...........................| 97
2...........................| 100
1 1/2 ......................| 102
1...........................| 105
1/2 ........................| 110
1/4 or less................| 115
____________________________|________________________________
 Footnote(1) When the daily noise exposure is composed of two or
more periods of noise exposure of different levels, their combined
effect should be considered, rather than the individual effect of
each. If the sum of the following fractions: C(1)/T(1) + C(2)/T(2)
C(n)/T(n) exceeds unity, then, the mixed exposure should be
considered to exceed the limit value. Cn indicates the total time of
exposure at a specified noise level, and Tn indicates the total time
of exposure permitted at that level. Exposure to impulsive or impact
noise should not exceed 140 dB peak sound pressure level.