Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

What Is a Watt?
A watt is a unit of measurement. This particular unit measures power, or capacity to do work. Watt's law (named for James Watt, an 18th-century Scottish engineer) states that one watt (1W) is how much power is in an electrical circuit where one ampere (1A), or amp, of current flows across a difference of one volt (1V).
Function
Using the equation P=V x I, where P = power in watts, V = voltage, and I = current, we can determine that an AC adapter rated at 65 watts with an output of 12 volts AC will have a maximum electrical current output of approximately 5.42 amps (12V x 5.42A = about 65W).

An AC adapter with a rating of 90 watts with an output of 12 volts AC will have a maximum electrical current output of 7.5 amps (12V x 7.5A = 90W).
Therefore, an AC adapter with a higher power rating (measured in watts) will be able to supply more current to a device it's connected to.
Practical Application
Since both AC adapters are capable of supplying the same voltage, it may seem like there isn't much difference between the two. However, the amount of current the adapter can supply should play a major factor in your decision as to which one you will use.

For example, a device that requires 12V AC and 6A will require an AC adapter that is rated for at least that much power (72W). Trying to use a 65W AC adapter will not provide enough power, because the AC adapter will not provide enough electrical current. In this case, the 90W power supply would be the appropriate choice to power the electrical device.

Similarly, since the power rating is considered to be a "maximum allowable power" indication, an AC adapter may be used to power devices that are rated at or below their power rating. If a device requires 12V and 3A (36W), either the 65W or 90W power supply may be used to power it.
Other Considerations
If an electrical device contains a rechargeable battery, an AC adapter that has a higher power rating will allow for faster battery charging: An AC adapter with a higher power rating will allow more electrical current to flow, allowing more of the spare electrical current to flow into the battery.

Cost may also be a factor. Usually, since AC adapters with a higher power rating must be capable of delivering more electrical current for the same amount of voltage, the components required to assemble an AC adapter must be more robust than that of a lower-power-rated AC adapter.

AC adapters with higher power ratings also give off more heat than adapters with lower power ratings. If excess heat is a consideration, an adapter with a higher power rating may not be suitable for the application.