Table of Contents
Entanglement is often cited as the phenomena that makes most apparent the distinction between classical and quantum. Unlike classical behavior, when two quantum systems become entangled one can no longer fully describe the state of one system without a description of its partner.
In addition to studying quantum entanglement to enrich our understand of quantum mechanics generally, we have recently realized the prospects of entanglement towards computation and communication [1,2,3,4,5,6]. Thus, much research is being done into the generation and preservation of entangled (as well as other) states. Amongst recent discoveries are systems that disentangle in finite-time (a phenomena known as Sudden Death) , systems that experience disentanglement for only an instant , and systems that always exhibit some degree of entanglement despite environmental influence (Always Living). 
In order to closely study entanglement, we first need to be able to quantify it. Although Bell's inequalities sharpened the distinction between classical correlations and quantum entanglement, there still remains no single means of quantifying entanglement.  Depending on one's purpose, one can choose from a variety of measures, each with individual benefits and drawbacks. Despite the absence of a single measure, much progress has been made into the study of entangled systems.
Section 2 of this website provides some brief background into entanglement measures generally an overview of some particular measures. Section 3 discusses some recently discovered phenomena exhibited by entangled systems.