what is sampling with and without replacement?

Answer 1

When you sample with replacement, you return the sample to the population pool, and each selection is independant. That is, the probabilyt of a given outcome is the same regardless of previous outcomes.
When you sample without replacement, you keep the sample out once it has been selected, and the steps of the experiment are no longer independant. That is, the probability of a given outcome can change depending on the previous outcomes.
This concept is illustrated in exercises where you select cards from a shuffled deck. For example, you want to know the probablity of selecting 2 kings. If you pick a king and put it back, the probabiliy of the next card drawn being a king, is still 4/52. However, if you do'nt put it back, the probabiliyt of the next card drawn being a king, is 3/52.

Answer 2

It is used in probability of an event happening. The probability of picking, say, a red marble out of a bag of ten marbles, 5 red and 5 blue would be 5/10, or 1/2. But the next marble pulled would have a 4/9 prob. of being red, if the 1st marble was not replaced (only 9 marbles would be left in the bag) Whereas the probability of the second marble being red would again be 5/10 if the first marble was replaced.