Places with a climate that is both comfortable to humans and able to support crops will have a larger population.  With the invention of climate controlled homes, people are setting in areas that once would have been sparsely populated.  Climate still plays a role in population.  It also plays a significant role in how that population lives.  For instance, some homes in the northern US do not have air conditioning because the temperature is rarely warm enough to need it.  They also import a lot of food because it is too cold for many crops.  Homes in some desert areas of the US have air conditioning everywhere.  They have air conditioned garages too because the heat is dangerously high.  

The places with the most temperate (that is the most comfortable) climate are going to have the most population, usually, as long as the place is otherwise easy to live in. So a flat, fertile, water-rich place with a good climate is where people will settle. People usually also settle near water, for ease of transportation, and the water affects the climate.

One connection between climate and population is that a larger population in the world today seems to lead to a warmer global climate.  A larger population leads to greenhouse gas emissions that lead to global warming.