the following are the reasons for sampling:
1. To bring the population to a manageable number
2. To reduce cost
3. To help in minimizing error from the despondence due to large number in the population
4. Sampling helps the researcher to meetup with the challenge of time.
In fact sampling techniques are of two categories viz (i)Random Sampling Techniques and (ii) Nonrandom sampling techniques. Then there are the mixed sampling techniques.
Simple random sampling
Stratified Random Sampling
Systematic Random Sampling
Quota Sampling and
Multistage sampling. the first one is the simplest form of Random Sampling
The Rest are Mixed Sampling devices. If further explanation is required ask individual questions.