During prayer the Muslim must avoid anything that invalidates his prayer. The invalidators of the prayer include:
Uttering words which do not include mentioning Allaah
Uttering a letter or two letters that carry a meaning while remembering that one is praying
Performing many moves that lasts for the duration of one Ra'kah which is one cycle
Eating, even if it is little, like a sesame seed, except if one forgets
Drinking while remembering that one is in the prayer. Even if it is little, like swallowing a drop of water on one's lips
Performing one move with the purpose of playing
Performing an excessive move such as jumping
Purposely adding an extra integral of action such as making sujood three times in one Ra'kah
Intending to interrupt prayer, in this case one's prayer becomes invalid immediately
Making the intention to interrupt prayer contingent on the occurence of an incident, such as saying in one's heart
"if my brother comes I willl interrupt my prayer and open the door for him". This invalidates the prayer immediately, even if one's brother does not come
Hesitating about interrupting the prayer
Completing an integral while the doubt is still persisting about one's intention during the opening takbeer, or doubting a long time about this intention.
For example: If someone recites the Fatiha while doubting about if one intended Dhuhr or Asr, one's prayer is invalid.
The prayer is also invalid when the time of doubt is done even if an entire integral did not pass. An example is if one recites the entire Surah of ___ while one doubts one's intention, then one's
prayer is invalid