Tooth colored fillings are also known as composites, because they are made of a composite of plastic and zirconia/silica. Composites were introduced in the 1960's, and have been improved continually in durability, color and appearance. The decay is removed, an adhesive that actually bonds to the tooth is applied followed by several layers of the tooth colored composite. Once the filling is in place, it is chemically hardened, or cured, for less than a minute.
Tooth colored fillings mimic the appearance of natural teeth. The composite material comes in a great number of colors (shades), and are matched to your individual tooth color. Most people want their teeth to look natural when they laugh, talk and smile. Because tooth colored fillings blend so well with the natural enamel of the tooth, they are virtually undetectable.
Tooth colored fillings now are harder and more durable than silver / amalgam fillings. A small filling could conceivably last your lifetime, and larger fillings can be expected to last many years. The factors that determine the lifespan of a particular filling include: the tooth (molars take more force than bicuspids), the size of the filling relative to the tooth (smaller fillings will usually be more durable than larger fillings), and how teeth are cleaned and maintained daily.
A new filling should feel natural, like your other teeth. After the anesthesia has worn away and you can feel your teeth and bite again, you should not notice anything particularly different. It should be smooth and comfortable. You can eat on tooth colored fillings right away (as soon as anesthesia is gone) compared to silver / amalgam fillings, which you cannot.