An Attempt to Explain the Menstrual Cycle in Plain Language

Period Facts:

Menstruation occurs only in humans and other closely related species (like chimps).  
Other mammals have estrous cycles in which the uterine lining is completely reabsorbed by the animal. (sounds nice!)
The average age of onset of menstruation is age 12.
When counting the cycle, the first day of blood flow is counted as Cycle day 1.  
The average duration of a cycle is 28 days but any cycle length between 21 and 34 days is considered normal.  
The average duration of flow is 4 days but anything from 2 to 7 days is considered normal.
On a typical 28 day cycle, ovulation occurs on day 14.  
The menstrual period is always exactly 14 days after ovulation.  The time from period end to ovulation is variable.
A woman is most likely to get pregnant on the three days preceding or the day of ovulation.

There are four major players involved in regulating a woman's menstrual cycle:

The HYPOTHALAMUS: an area of the brain just above the pituitary gland.
The PITUITARY GLAND: a little gland that hangs from a stalk just behind the eyes.
The OVARIES: a pair of glands, each about the size of a walnut that most typically sit
     behind the uterus in close proximity to the fallopian tubes.
The UTERUS:  a muscular cavity typically the size of a pear that inhabits the space between
     the bladder and the rectum in the female pelvis.

There are five major hormones involved in creating a cycle:
(Hormones are messengers that carry messages/instructions from one tissue to another)

GONADOTROPIN RELEASING HORMONE (GnRH)
FOLLICLE STIMULATING HORMONE (FSH)
LUTEINIZING HORMONE (LH)
ESTROGEN
PROGESTERONE

What Is Happening During a Cycle?

***In the first half of the menstrual cycle--also known as the Follicular Phase:

The HYPOTHALAMUS is signaling the PITUITARY GLAND to produce FSH and LH--it does this by secreting pulses
of GnRH every hour. It also secretes a continuous small amount of GnRH, but the pulses are very  important.
The PITUITARY GLAND is producing LH which instructs the ovaries (theca cells) to produce estrogen.
The PITUITARY also produces FSH which tells the follicles (tiny cysts containing eggs) to start to grow.
Approximately 30 follicles will begin to grow.  By cycle day seven, the dominant follicle is established
and continues to grow while the other follicles degenerate. The dominant follicle will ultimately be the
only egg ovulated in this cycle. (Ocassionally there is more than one).
The OVARIES (in addition to making the follicle) are busy producing estrogen which tells the uterine lining (endometrium) to thicken and grow.
The UTERINE LINING after menstruation is very thin, only about 1-2mm thick, and will look like
a single, thin white line on ultrasound.

***At mid-cycle:

The dominant follicle is producing approximately 500 mcg of estrogen daily, it is also producing a
small amount of progesterone.  The follicle is now about 2.0 cm in diameter, and easily seen on ultrasound.
It has the appearance of a small dark fluid-filled cyst.

When estrogen levels have been this high for two or more days the LH surge is elicited--the pituitary gland
secretes a large bolus of LH. This LH surge is detectable in the urine and is the basis for ovulation predictor kits.  

There is also an FSH surge.  Rapid peaking of these two hormones signals the ovary to initiate rupture of the
follicle and ovulation of the egg.  Ovulation occurs within 24-48 hours after the LH surge.

If sperm are available at this point, now is their chance. If a sperm penetrates the zona pellucida ( a special membrane which surrounds the egg and permits only passage of Human sperm ) then fertilization occurs; if no one gets lucky, the second half of the cycle occurs.

The egg then goes on its merry way down the fallopian tube, while the follicle
from which it was expelled becomes the corpus luteum cyst. This cyst too, like the follicle is visible
on ultrasound, but has a very different appearance.

***In the second half of the cycle--also known as the Luteal Phase:

The HYPOTHALAMUS is sending out pulses more slowly--it is producing a pulse of GnRH about every
2-3 hours. Again, this is signaling to the pituitary gland to produce LH.
The PITUITARY GLAND  continues to produce higher levels of LH in the second half of the cycle than
it did in the first half.  High levels of LH signal to the corpus luteum cyst (the cyst the was produced
when the follicle burst) to produce progesterone.
The OVARY continues to produce progesterone in increasing quantities for about 7 days after ovulation.
It is also producing estrogen in high quantities. If pregnancy has not occurred, these high levels
of progesterone and estrogen signal the pituitary gland to turn off production of FSH and LH.
It is believed that the high levels of progesterone in the ovary which produced the
follicle prevent maturation of new follicles in the same ovary--in other words, if right ovary ovulated this month then left ovary will ovulate next month.
The UTERINE LINING, under the influence of high progesterone levels, has stopped thickening and
started maturing in preparation to receive the embryo (if any). One thing the uterus lining tissue
does is start to store glycogen (sugar) within the lining cells. This sugar will be released into the uterine
cavity to nourish an embryo (if any) until it can implant in the uterine wall. At this stage the uterine lining has a trilaminar (three lines) appearance on ultrasound and is about 12mm thick.

***Menstruation:

If no conception has occurred, exactly 14 days after ovulation the menstrual period will begin.
During menstruation, the uterine lining is sloughed and comes out as the menstrual flow.  
The uterus in effect, cleans itself down to the "roots" and the whole process begins again.
If pregnancy occurs, menstruation does not, but that is another story!

So, to simplify even further and summarize:

The cycle starts with menstruation. In the first half of the cycle, the ovaries are producing
estrogen which tells the uterine lining to grow.  The follicle is developing.  
At mid cycle ovulation occurs.
In the second half of the cycle the egg is sent down the fallopian tube while the cyst cavity left behind
begins to produce progesterone.  Progesterone tells the uterine lining to stop growing and start
storing up sugar for the embryo. If no conception occurs hormone production decreases and
the uterine lining is shed in the form of the next period.
Next month the follicle develops within the oposite ovary and the whole process starts over.  

Of note the whole process falls apart if no ovulation occurs, but that is another story too!

Lots more information is available at:

http://www.womenshealth.gov/faq/menstruation.cfm
http://en.wikipedia.org/wiki/Menstrual_cycle