Autism is a condition you are born with, you cannot catch it. It is a triangle of impairments in the following: communication, social interaction and imagination. Examples of how these can affect someone:
Communication. People with autism have difficulty in communicating with others. Their autism may be so profound that they are unable to learn to talk, read and write. They may find communication so stressful that they choose not to communicate, even when they have the ability. An example is elective mutism. They may have the ability to learn a way of communicating specific to them, sign language (usually makaton), key words, flash cards etc. some high functioning individuals can read, write and speak to the same level as people without a disability, but still struggle due to difficulties with social interaction.
Social interaction. People with autism have great difficulty in reading facial expressions and body language, in fact it's downright confusing. Everything is taken literally, so if you say it's raining cats and dogs, they will expect to see animals falling from the sky. So concepts such as humour, sarcasm etc. are very difficult if not impossible for them to understand. Social norms are lost too. This is why someone with autism may not understand that it's not appropriate to strip naked in public for example.
Imagination. People with autism tend to lack imagination. This is why routine is so important. Routine provides familiarity and therefore comfort and security. The unknown is frightening. So thus is why someone might watch the same movie scene over and over or have to have everything in a certain place.
That is a very generalised view coming from working with people with autism for 7 years. Everyone is very different, and their autism will present behaviours, challenges and needs individual to that person. Having autism is being on the autistic spectrum, from very low functioning to high functioning.
ADHD is again something you are born with and do not catch. It stands for Attention deficit hyperactivity disorder. In short, it causes inability to concentrate and hyperactivity.