A fard `ayn (individual duty) is an act that is obligatory for Muslims individually - each will be rewarded for performing it, or punished for failing to perform it. An example is the five daily prayers, for which Muslims are individually responsible.
A fard kifaya (collective duty) is an act that is obligatory for the Muslim community collectively - if it is sufficiently carried out by some members of the Muslim community, then other Muslims do not have to perform it; but if nobody takes it upon himself or herself to perform the act on behalf of the community, then all Muslims have failed (and will be punished). An example is the prayer performed at a burial: it is obligatory that someone attend the funeral to perform it, but as long as someone does it, others are not obligated to attend.
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