Knowledge, skill and education requirements vary between agencies. One way to get an idea of what is required is to look at the job postings on the Crime Scene Investigator Network. Also I suggest you look at the following page: How to Become a Crime Scene Investigator. It should give you an idea of what is needed to do to become a CSI or Forensic Scientist.
Job duties are different between agencies. Some positions only work in the field documenting scenes and collecting evidence. Other positions may only work in a laboratory. But there are positions that do some of both. Most Technicians that work in the field will process some of the evidence at the station but will send other evidence (especially biological evidence) to the crime laboratory. And some Criminalists who work in the crime laboratory will occasionally go into the field to assist with evidence collection. Again, it depends on the agency.
There can be specialization in the jobs—especially in the crime laboratory. Criminalists can be assigned to DNA, trace evidence, firearms, fingerprints, etc. Technicians in the field are usually generalists. They do everything (photographs, diagrams, evidence collection, etc.). But some may have special training in certain aspects of crime scene identification, such as blood spatter analysis and crime scene reconstruction.
Daniel Martinez wrote
What knowledge is required for a Forensic Technician? Also, do the Forensic Technician go in a lab and at the scene of the crime? What do they specialize in?