Do you live in the U.S.? If so, ask your pediatrician about your state's birth to 3 program. When my daughter was 24 months old, her spoken vocabulary was 0 words. She would not point, would not respond to her name, did not respond to any commands or anything including things like "let's go get ice cream". She was diagnosed with PDD, but only recently, and she's almost 3 years old. She also has other behaviors, such as repetitive behaviors, OCD like behavior, rituatlistic behavior, and after she started speaking, she is very echolalic (repeats without understanding sometimes). Like, when she falls down, we used to ask her "Are you okay?" Well, now when she falls down she says "are you okay?" She doesn't understand all the time that things are questions, and I think she thinks when she falls down, that's what you're supposed to say. The other week, at the playground, a girl was shouting for her mommy, and my daughter thought it was a game, so she started shouting "mommy" right after the girl was calling out. It frustrated the girl to no end because she thought my daughter was making fun of her. I am not sure if she was trying to help teh girl get her mommy or thought it was an entertaining game. I'm thinking she thought it was a game because she was laughing her head off every time they shouted mommy.
i would not worry immediately that it's autism. Lack of speech and repeating speech also occurs with speech and language delays. You really need to address these issues with the pediatrician. 100 words at age 2 is a good amount for vocabulary. 2 is the average age when kids start to combine 2 words together. A speech therapist may be able to help, but if your child doesn't qualify yet, she can be reevaulated a few months from now. If you are in the U.S., evaluations for the Birth to 3 programs are free. Therapy in the Birth to 3 programs are government subsidized, and generally are either free or a low cost copay (our copay was $3 per 1 hour session). Many states, the therapists either come to your house, or if you have daycare, they will even go to the daycare provider. Some states you have to travel to the therapist. It really all depends on what state you live in. Again your family's pediatrician should have information to give you. Not sure what to advise if you live outside of the U.S., other than check with the pediatrician to see what is available.
If you click on the Health Page icon on the top right hand corner of this forum page you can go into The Behavioural Characteristics Behind the DSM IV Diagnostic Criteria for Autism. This is the clinical criteria the professionals use to assess whether a child is on the spectrum or not. Parents have also posted examples of their child's behaviour that fits the criteria so that will give you an idea of what the professionals are looking for.
From your post you are saying your daughter is 2 years old and is using single words. She is using language to get things she wants. But she doesn't use language in a social chatting type of way.
As she is only 2 these are social skills that may begin to emerge. But to be sure it might be advisable to go to the GP or Paediatrician and say that you think she is behind with her speech and communication and ask for her to be assessed by a Speech and Language Therapist. You can ask for one that is experienced in assessing children on the spectrum.
The fact that her response to verbal communication is variable could also be an indication of a speech or sensory disorder which may or may not be part of an autistic spectrum disorder. But I am a parent and not a professional.
My son is on the autistic spectrum and I too noticed that his speech wasn't developing at the rate that other childrens were. He seemed to be using language as your daughter is. Using one/two word combinations with pointing, gesturing. Sometimes he appeared to understand what I said. At other times he gave an answer that was completely unrelated to the question I had asked him. At other times he appeared deaf.
If your daughter also 'appears deaf', this could be a sensory disorder or could be an attention or focus issue whereby when her attention is on one thing she effectively screens out any other sensory stimulus and therefore is unaware of you speaking to her.
Don't be afraid of having her assessed and observed. If it isn't autism she won't get a diagnosis. It could be a speech delay, or sensory disorder, or auditory processing disorder etc etc. But it is advisable to start the assessment process now so that any help or supports she needs in nursery or school are in place by the time she begins there.
My daughter is 25 months old. I forgot that bit of information.