When you have a situation of cold air being replaced by warm air, it's not too uncommon to get a bout with freezing rain in spots.
Many people might think that ice pellets are "freezing rain" but no, freezing rain is a specific type of precipitation, and when there are copious amounts, it can actually cause more problems than snow.
Freezing rain is caused when you have a warm mass of air in the middle altitudes between the ground and the cloud deck, followed by a mass of freezing air near the surface.
When the precipitation falls from the cloud, it will start as snow. As it encounters the warm air, it will melt into the usual rain. But right before it reaches the ground, it enters the area of below-freezing air and cools to right around freezing -- or perhaps just below, a process called "supercooling."
But once that supercooled raindrop hits an object, it instantly freezes into ice. Thus, a short period of freezing rain can turn any surface into a sheen of ice -- including roads, sidewalks -- and driveways.
Here is an example I found on YouTube of how freezing rain can make what appear to be bare and wet streets a bit more treacherous:
Freezing rain, also know as "ice storms", also can cause massive power outages as the weight from a thick coating of ice can topple power lines by themselves, or knock trees into power lines. Ice storms are quite common in the south when warm air from the Gulf intermixes with cold air from a Canadian-borne storm.
One of the greatest ice storms hit in upstate New York and across parts of Ontario and Quebec in 1998, with ice accumulations as great as 2-4" thick.
Around the Northwest, ice storms are rare in the Puget Sound region, but can put down 1/4-1/2" glaze from time to time. The most common areas for freezing rain are near mountain outflow areas such as Whatcom County near the Fraser River outflow and along the Cascade foothills where cold air will funnel in from Eastern Washington. Also, valley areas, such as the Kent Valley, can get freezing rain as cold air is heavier than warm air and tends to pool in valleys. Thus, that situation can provide conditions conducive to freezing rain.
For Tuesday night's storm, freezing rain is a possibility in those aforementioned spots around the Puget Sound area, but it is expected to be brief and before sunrise Wednesday.
Freezing rain is much more common in the Portland area where cold air frequently gets funneled in from Eastern Washington and Oregon via the Columbia River Gorge and can make for an extended period of freezing rain in the city.
Then, what is sleet?
Sleet is snow that begins to melt as it reaches the ground, but doesn't melt completely. This usually happens when it's around 33-35 degrees at the surface. Sleet usually doesn't cause as much of an issue.