Berachos, Chapter 4, Mishnah 3. "Rabban Gamliel omer: bechol yom mispallel adam shemoneh esreh" - a person is obligated to recite a prayer of eighteen berachos every day. In practice, a berachah was added after the time of the Mishnah - "Velamalshinim" - so that the actual number is nineteen berachos, and nevertheless we still customarily call this prayer "Shemoneh Esreh."
"Rabbi Yehoshua omer: me'ein shemoneh esreh":
Rabbi Yehoshua disagrees and holds that a person is not obligated to recite the full text of Shemoneh Esreh every day; rather, he may say "me'ein shemoneh esreh" - an abbreviated version of the prayer. The Gemara records a dispute as to exactly what this version is, but according to all opinions six berachos are said in full: the first three berachos and the last three berachos:
The first three: Avos, Mechayeh HaMeisim, and HaKel HaKadosh.
The last three: Retzei, Modim, and Sim Shalom.
The dispute in the Gemara regarding the middle berachos:
One opinion: he says each and every one of the middle berachos, but he says them in an abbreviated form. This is what Rabbi Yehoshua means.
A second opinion: he says one long middle berachah, called 'Havineinu.' This berachah incorporates all the themes of the middle berachos, yet it is nonetheless only a single berachah, concluding with "Baruch Atah Hashem shome'a tefillah" - the conclusion of the last of the middle berachos.
"Rabbi Akiva omer":
According to Rabbi Akiva, it depends on the ability of the one praying. "Im shegurah tefillaso befiv" - that is, if he is able to pray fluently - he must say the full Shemoneh Esreh, since that is the obligation whenever a person is capable of meeting it. But if because of some circumstance he is unable to say the complete prayer, he may say "me'ein shemoneh esreh": either the text of 'Havineinu' or the abbreviated middle berachos.
When 'Havineinu' is not said:
The Gemara notes that 'Havineinu' can only be said during the summer months, because the structure of the berachah leaves no room for additions:
"Veten tal umatar livrachah" - the addition made during the winter months.
"Atah chonantanu" - the addition made on Motza'ei Shabbos, after Shabbos has ended.
At these times, then, a person has neither the opportunity nor the ability to fulfill his obligation with the text of 'Havineinu.'