Hello fellow PersonalityCafeers,
since there are repeated questions about limerence, I decided on writing a series of articles about it during the next days and weeks to come :).
So let's get started right away:
Part 1 - What is this thing called limerence?It is not love. It is the force of evolution expressed as the compulsion for the particular, the particular one above all others. Often, it is called love...
Introduction and characteristics of limerence
The term "limerence" was introduced by psychologist Dorothy Tennov in her work on romantic love, especially Love and Limerence: The Experience of being in love.
The term is "used to describe an involuntary state of mind which seems to result from a romantic attraction for another person". 
The basic concept is the scientific approach to the field of romantic love. 
The object of your passionate desire is oftentimes called Limerent Object or (in short) LO.
Very typical for limerence is the obsessive need to have one's feelings reciprocated, along the following basic components :
- intrusive thinking about the LO
- acute longing for reciprocation
- dependancy of mood on your interpretation of LO's actions with respect to the probability of reciprocation
- inability to react limerently to more than one person at a time (with an exception at low levels of limerence)
- fear of rejection
- increasing discomfort and feeling of tension when uncertainty increases
- acute sensitivity to any act, thought or condition that can be interpreted favorably
- an extraordinary ability to devise or invent "reasonable" explanations for why the neutrality that the disinterested observer might see is in fact a sign of hidden passion in the LO
- the feeling of ecstasy when reciprocation is evident or interpreted that way
- deep depressive mood after rejection seems evident or hope is decreased 
- intensification through adversity
- a general intensity of feelings and emotions that leaves other concerns in the background
- crystallization of the LO
It has to be noted that limerence is, although very common among humans, not an universal state which applies to everyone. In contrast to limerents there are also the Non-limerents. 
Hence, according to limerence theory, there are three possible (romantic) bonds: 
- Nonlimerent-Nonlimerent / Affectional bond: relationships in which neither partner is limerent
- Limerent-Nonlimerent bond: relationships in which one partner is limerent
- Limerent-Limerent bond: relationships in which both partners are limerent
The bulk of relationships are limerent-nonlimerent bonds which are characterized by unequal reciprocation.
What increases limerence ?
The limerent reaction feeds upon the right mixture of uncertainty and hope.
The objective that the limerent person pursues, as is clear in the fantasy that occupies virtually very waking moment, is a "return of feelings". The ecstatically blissful moment, toward which the long fantasies progress and in which the LO gives what the limerent accepts as clear indication that the limerent goal has been achieved.
Uncertainty about LO's true reaction is an essential aspect of limerence. Removal of uncertainty is another main goal, and since that desire is so unrelenting, so imperative, the limerent continually searches for the meanings underlaying events.
Whenever hope or uncertainty is increased, the limerent symptoms become more severe.
The duration of limerence
Tennov finds that both states, the state of limerence and non-limerence tend to endure, although the reasons are different. Limerence is sustained because one of its aspects is the desire for limerence itself. This desire transcends the feelings directed toward a particular LO and persists even after preoccupation with that LO has diminished to a low level.
In comparison to the non-limerent, for which the issue concerns more "rational" goals, the limerent it is more of a passionate, involuntary yearning.
For the non-limerents a relationship may be very much desired, but failure of attainment does not produce the type of intense misery that may be unique to limerence. 
The general duration of limerence is hard to estimate, as it relies on a variety of individual factors.
It can range from a couple of days up to a lifetime. Tennov's estimation for the average duration of limerence is two years , although other estimates put it at 6 months to three years.
Physical effects of limerence
The physiological correlations of limerence are heart palpitations, trembling, pallor, flushing, pupil dilation and general weakness. Awkwardness, stuttering, shyness, confusion predominate at the behavioral level. There may be sickness, headaches, etc., and loss of appetite, dizziness and passing out: the latter very rarely happens, but can take place when the person is deeply in love with the limerent object.
A condition of sustained alertness, a heightening of awareness and an enormous fund of energy to deploy in pursuit of the limerent aim is developed. The sensation of limerence is felt in the midpoint of the chest, bottom of the throat, guts, or in some cases in the abdominal region. This can be interpreted as ecstasy at times of mutuality, but its presence is most noticeable during despair at times of rejection.
Limerence and infatuation - where is the difference?
One of the main differences is the level of anxiety in regard to fear of rejection.
While being in a limerent state the fear of rejection can be overwhelming and actually prevents the limerent from interaction with the LO. Every behavior and sometimes even thought is constantly checked against whether it would make the limerent more favorable in the eyes of the LO.
This is also one essential reason for the fantasy part of limerence. Fantasy gives the limerent transient relief from the limerent desire and longing, but is always somehow rooted in reality.
Most of the times it reaches its height in a situation where the limerent perceives the limerent goal to be attained (as mentioned before).
This leads to severe insecureness and shyness in the presence of the LO, especially when in the imagined situation.
What role plays sexuality in limerence?
According to Tennov's work, awareness of physical attraction (even if consciously suppressed) plays a key role in the development of limerence.
It rather depends on the meaning attached to sex - the limerent meaning.
Since the correlation of sexuality and limerence is rather complex, it will be discussed in a separate article.
Happily looking forward to any comments, more articles to follow :).
 Limerence - Wikipedia, the free encyclopedia - Introduction - retrieved 23/04/2011
 Limerence - Wikipedia, the free encyclopedia - Characteristics - retrieved 23/04/2011
 Tennov, Dorothy - Love and Limerence, 1999, p.23/24
 not part of 
 Tennov, Dorothy - Love and Limerence, 1999, inlay
 Tennov, Dorothy - Love and Limerence, 1999, p. 110ff.
 Human bonding - Wikipedia, the free encyclopedia - Pair bonding - retrieved 23/04/2011
 Tennov, Dorothy - Love and Limerence, 1999, p. 130-140
 Tennov, Dorothy - Love and Limerence, 1999, p. 57ff.
 Tennov, Dorothy - Love and Limerence, 1999, p. 140ff.
 Tennov, Dorothy - Love and Limerence, 1999, p. 142
 Limerence - Wikipedia, the free encyclopedia - Physical effects - retrieved 23/04/2011,
 Limerence - Afraid of Rejection in Love - How to Get Over It? - The effects of limerence - retrieved 23/04/2011