All children develop at different rates and their capabilities are varied from birth. We have early walkers, late talkers, super gifted readers, computer whizz kids and so on. Every single child on the planet is special and completely unique with their own strengths and their own weaknesses. Still, parents do like to know, as a rule, what's "normal" and whether their child is reaching all of their milestones on time. Speech is definitely one which is most often discussed between mum's and dad's, and while there is usually no need for concern over a slight delay, it is a factor that can point to other problems when a child has previously shown no concerns at all.
Autism, for example, is often picked up on because of a speech delay. Milestones before this can all be met at a normal, often faster pace, than you would expect of any other child, so it is important to take notice of where your child should be in terms of their speech development - that way, problems can be addressed early and any further complications can be picked up on quickly. Of course, you must take everything you do read with a pinch of salt, and remember that your own instincts are just as valuable as a research paper.
The study which took place at the child study institute at Bryn Mawr College determined that children should know 50 words by the age of two, and they should be starting to link words together from the age of two. They perhaps wont be clear and concise but a vocabulary of 50 words should be there. If there is less, it may not necessarily be a "problem" as such as all children do develop at different rates, but you should perhaps start to chart your child's progress to check they are continuing to develop their language skills.
To allow for speech delays and other attributing factors as to why a child may have less than 50 words speech, the director of the Child Study Institute, Leslie Rescorla,was asked to select the 25 words that children of 2 should know (as opposed to 50) from a list of the 351 most commonly used words by children of that age group. She determined the following list:
Mommy Speech Therapy.