The external links lead to sites containing more information/examples of the things I have mentioned.
Autism usually refers to autism spectrum disorder (ASD), which is a group containing autistic disorder, pervasive developmental disorder not otherwise specified (PDD-NOS), Asperger’s syndrome (AS) and, depending on what you are classifying, childhood disintegrative disorder (CDD) and Rett syndrome.
This is the proposed diagnostic criteria for ASD (read the ‘proposed revision’ and ‘severity’ sections)- http://www.dsm5.org/ProposedRevisions/Pages/proposedrevision.aspx?rid=94
Autism is a severe, lifelong developmental disorder that consists of a large spectrum of ability and disability ranging from severe ( http://www.youtube.com/watch?v=PPWL5yimhyg&feature=g-hist ) to mild ( http://www.youtube.com/watch?v=Ludzl-UEcPg&feature=related ). Functioning levels (high to low functioning) are also used, these take into account the presence/severity of intellectual impairments, epilepsy, self injurious behaviour etc.
Many individuals with ASD do have trouble regulating their emotions, and this can lead to them losing their temper. Most individuals with ASD also have sensory processing disorder ( http://www.sensory-processing-disorder.com/ + http://www.youtube.com/watch?v=plPNhooUUuc ). Sensory overloads caused by sensory processing disorder (SPD) or can strongly resemble tantrums, but the person experiencing it really can’t help it. SPD may also cause someone to not want to go into certain situations (eg. Not wanting to put on their clothes due to tactile hypersensitivities) which may resemble oppositional behaviour.
People with ASD have impaired communicative abilities. In a mildly affected individual this may be limited to impairments in nonverbal communication (such as an inability to read facial expressions and not understanding concepts like personal space) and atypical speech. In a severely affected individual this could consist of no speech, some learn to use varying amounts of sign language or electronic communication devices (such as iPad apps specifically designed for individuals with limited or no speech). Some individuals have no communicative abilities whatsoever.
People with ASD also have significant impairments in social skills. Some individuals may wish to have friends but be unable to make them (without help), whilst others have no desire for friendships.
People with ASD have repetitive and restrictive behaviors. This can take the form of stereotyped movement or speech (echolalia, rocking, hand flapping, self injurious behaviors) ( http://www.youtube.com/watch?v=15C79nPbcx8&feature=related ), these are theorized to be related to the SPD. People with ASD are very reliant on strict routines, and become very distressed when their routines are broken ( http://www.youtube.com/watch?v=pX9MZ2yg6-U ). These routines can consist of things such as: wearing the same clothes every day, taking the same route, sitting in the same spot, even eating the same food.
A few people with high functioning ASD are very talented in the areas of mathematics, science and technology, but a significantly higher proportion have immense difficulty in these areas. Comorbid learning disabilities (dyslexia, dyscalculia etc.) are common. Intellectual impairments affect 40% of people with ASD (but, obviously, nobody who is high functioning). All the known genetic causes of ASD cause comorbid intellectual impairments.