Tagliatelle are thick ribbons (about 1/4" wide) of semolina pasta used with heavy meat or cream sauces. Tagliatelle looks similar to fettuccine and in fact Giuliano Bugialli, (cookbook author, born in Florence, Italy) says that really the two are one and the same. Bugialli explains that the difference is simply regional variance or name.
Another regional variance you may see is that eggs may be included in the pasta ingredient list. The taste is very similar and most likely you would not even be able to tell the difference between the two. If you are looking specifically for tagliatelle pasta I would not get bogged down trying to find one version over the other.
Two classic preparations for using tagliatelle include Bolognese ragu meat sauce or served with a black truffle (al tartufo) cream sauce.
This is a pretty fast cooking pasta.
Add the dry tagliatelle to a pot of boiling, salted water.
Cook for 7-10 minutes, drain and serve. Because manufacturers may vary the width or thickness of their pasta make sure to follow the package instructions.