What invalidates prayer in Islam?

Losing your purity

Prayer needs wudu from start to finish. The Prophet ﷺ said that the prayer of someone who breaks their wudu is not accepted until they make wudu again (Sahih al-Bukhari 135). So if you pass wind, or anything else happens that breaks wudu, the prayer ends there. Leave quietly, renew your wudu and pray again. See What breaks wudu?

Don't let doubt trick you, though. If you only think something happened, the Prophet ﷺ said not to leave until you hear a sound or notice a smell (Sahih al-Bukhari 137). Certainty isn't cancelled by a doubt.

Talking, eating and laughing

The Prophet ﷺ said that ordinary speech has no place in prayer; it is only glorifying Allah, saying Allahu akbar and reciting the Quran (Sahih Muslim 537). So:

• Talking on purpose (answering someone, chatting) breaks the prayer. Most scholars excuse a word said by someone who forgot they were praying or didn't know the rule.
• Eating or drinking on purpose breaks it, by agreement.
• Laughing out loud breaks it. A smile does not.

Missing a pillar or a condition

Some things are the backbone of the prayer: standing (if you can), the opening takbir, reciting al-Fatiha, bowing, prostrating, sitting, and doing each calmly. A man once rushed his prayer, and the Prophet ﷺ told him three times, "Go back and pray, for you have not prayed," then taught him to be still in each position (Sahih al-Bukhari 757). Leaving out a pillar on purpose, or turning your chest away from the qibla, or uncovering your awrah for a noticeable time, makes the prayer invalid.

Lots of extra movement (like doing something else entirely) also breaks it. Small movements, like scratching or fixing your scarf, don't. See What if my phone rings while I'm praying?

What does NOT break your prayer

• Wandering thoughts (try to refocus; the prayer still counts).
• Forgetting a minor part: fix it with the prostration of forgetfulness.
• A small stain you didn't know about. See Can I pray with a stain on my clothes?
• Crying out of humility, or a cough or sneeze you couldn't hold back.

The schools differ on some details, such as how much movement is "a lot" or whether a forgetful word counts. The Hanafis are stricter on speech, while the Maliki, Shafi'i and Hanbali schools excuse a little said by mistake.