Explanation of Some of the Nullifiers
"Whatever exits from the two private parts": Everything such as urine, feces, sperm, thin liquid that has no smell, fluid emitted after urination, gas, stones, blood, worms, menstruation, and post-natal bleeding.
"The exiting of big amounts of impure substances from other parts of the body": The correct opinion is that is does not nullify the ablution unless it is similar to urine or feces.
"Unconsciousness due to sleep or other than it": Sleep itself does not nullify ablution except that it is assumed that gas may come out. So, if one is certain that nothing has come out then s/he still has ablution.
"Touching the private parts, front or back, without a barrier": Ibn Taymiyyah – may Allah have mercy on him- says that it is recommended to make ablution after touching it and not a must.