The luteal phase is the day after ovulation and runs through the remainder of a woman's cycle.   It is also frequently referred to as "days past ovulation" or "DPO".  During the luteal phase a women produces progesterone increasing her temperature.  The higher temperature acts like an incubator which is important in maturing a fertilized egg if conception has occurred. 

Luteal Phase Length
The length of the first phase (pre-ovulatory) can vary within a woman's cycle - but the luteal phase usually remains the same.  Ovulation can be delayed by a number of factors....stress, illness, medication, heavily increased activity, etc.  Even if you do not ovulate around the normal time period -  in knowing the length of your luteal phase you are aware of exactly when your menstrual period is due.  This discounts the belief of those say they always have "late periods".  If they were charting they would realize that actually it was just their ovulation being delayed and they were not late at all.  A normal luteal phase is approximately 12 - 16 days and needs to be at least 10 days for implantation to take place.  A short luteal phase can cause fertility problems making it difficult for a women to get pregnant.

Identify Pregnancy
Since a woman's luteal phase is very consistent you may watch the length of your luteal phase to identify success in conception.  For example, if your luteal phase is usually 12 days and never more than 13 and you are at day 15 of your luteal phase and menstrual period has not arrived, there is a very good possibility that pregnancy has occurred.