Dry skin is a skin type (this is what we are born with) and is from a lack of oil and will be dry all year round not just seasonally. When you have a true dry skin type, you have minimal or no oil production in the skin.
Dehydrated skin is a skin concern that can happen to any skin type (Dry or otherwise) due to a lack of moisture (water) in the skin.
Dehydration is caused by external factors such as sun exposure, harsh or wrong product usage or treatments, hot showers and seasonal changes.
When your skin is dehydrated, you get a feeling of tightness, and you can also experience sensitivity.
It's important to note that because every skin type can become dehydrated, even oily skins can experience dehydration. Which can lead to an overproduction of oil (then leading to breakouts) as the skin tries to overcompensate for the lack of water.