A sample where the units are selected based on easy access/availability. The disadvantage of this technique is that the sample may not be representative of the population.
Taking items for the sample in a way that ensures that any unit in the population has an equal chance of being selected. This is intended to ensure that the sample is not biased by the method of collection. Bias may be inadvertently introduced by selecting items that are easy to collect, or accessible.
A portion of units taken from the population which is representative of the population for the purpose of the intended analysis.
The characteristics of the sample must be uniform and representative of the population. Suppose that a sample was taken from a batch of concrete that had not been properly mixed. The properties of the sample would depend on where in the concrete it happened to be taken from; it would not be representative of the population in general.
An alternative to random sampling. The population is split into various categories of items that share common characteristics, items are then collected at random from each category, in proportion to their presence in the population. This may be fairer than random sampling, given the practical difficulties of random sampling. There is also the possibility that with the vagaries of random sampling that some categories are not represented.
