A homeotherm is an organism which is able to survive only when the internal body temperature is within a narrow range. If their body temperature increases to higher levels or falls to lower levels the enzymes in the body are not able to function and this leads to the death of the organism. Examples of homeotherms include mammals, birds, etc.
Poikilotherms on the other hand can survive with their internal body temperature varying over a wide range. They are able to withstand a much larger variation in external temperature that affects the internal body temperature. Examples of poikilotherms are fish and reptiles.
This ability of homeotherms and poikilotherms related to having to maintain a constant body temperature has its own advantages and disadvantages. Some of which are that homeotherms need to use less resources as their body enzymes are highly efficient due to the fact that they need to operate in a narrow temperature range. This allows these organisms to grow larger and develop faster. Poikilotherms on the other hand use up a lot of resources producing inefficient enzymes which can operate over a wide temperature range.