Jean-Baptiste Lamarck was a French scientist during the early 19th century who developed a different theory of evolution before Charles Darwin.
Lamarck's theory involved two main ideas which were:
Any features that are improved through use by an organism are passed onto their off-springs.
Any characteristics that are used by an organism become bigger and stronger over time whereas ones that are not used will eventually disappear.
As technology and knowledge in this area have improved, we now know that this type of inheritance is not possible in most cases. Lamarck's theory suggests that effectively all organisms would become very complex and all simple organisms would disappear.
Lamarck's theory is often linked to the giraffe's neck becoming longer over time so they can eat the higher up branches. Lamarck believed that the giraffe used its neck and stretched it to get the higher up leaves and since it used this characteristic it got bigger (elongated in this case) and this was then passed onto further generations, progressively elongating the giraffe's neck.
Nowadays we believe that Lamarck's theories were wrong but things like the giraffe's neck elongating over time can happen because of things like mutations.