The Amidah is a prayer containing 18 benedictions. Nowadays, however, it has 19 benedictions, as during the Temple times a paragraph was added to curse those who informed on the Jews. Those informers would be unable to recite the paragraph out of morals, and would be spotted as informers.
The Amidah is changed on the occasion of a New Moon or festival, each event with its own small change. The Amidah is always recited standing, and parts of it may be chanted (although not out loud unless there is a minyan (traditionally the ten men required to pray)).

Quotations from the Hasidic masters on the Amidah:
Why are we told to recite the verse "O Lord, open my lips and let my mouth declare Your praise" before our most sacred prayer?
Like banks to a river, lips form the outer edges of human speech.
We pray that G-d may release us from those limits, so that our mouths may declare His endless praise.

As a person begins to pray, reciting the words: "O Lord, open my lips and let my mouth declare Your praise," the Presence of G-d comes into him.
Then it is the Presence herself who commands his voice; it is she who speaks the words through him.
One who knows in faith that all this happens within him will be overcome with trembling and with awe.