Many people confuse this and use the terms synonymously. BOTH are meant to be skin tight or to fit closer to the skin.
Are made of a thinner material and most of the time are made without a leg seam. They can come in footed or footless and are supposed to be worn under something else (anything that will cover the top part. The "patch" between the legs is called a "gusset" and incorporated for a better fit and also allows the tights to be worn without underwear as the gusset is often made of a breathable cotton and is hygenic. They will often be less opaque (see through) as the material stretches by bending the feet, knees or butt. They will also easily reveal any panty line from undergarments.
LEGGINGS: also known as YOGA PANTS (not very accurate due to varying styles of pants made for yoga)
Usually made out of a thicker material and most of the time they are fully opaque (not see through). There is always a seam on the inside of the legs and perhaps elsewhere (outside, in front or in the back). Most of the time, they are footless. Footed leggings do exist! They can be worn as pants. They tecnically are "pants" if you read the definition of "pants" from multiple sources. Leggings are very durable on the most part due to their thicker fabric. They are usually appropriate to wear on their own as they are usually fully opaque and do not reveal any undergarments, whereas tights are usually more see through due to a thinner fabric. While there are tights that are more opaque than leggings, they are generally thinner and have more stretch due to their fine fibers, but as a result, lower durability and easily stretched out to become more see through.