The difference between a veterinary techician and a veterinary assistant are the education and credentialling requirements.
No state in the US has any educational requirements for working as a veterinary ASSISTANT. This is an entry level-position in a veterinary facility and training is generally done on the job. Because most training is done on the job it is often very cursory and lacks the depth and breadth of a formal education. Veterinary assistants are generally taught the basic how-to but not the why or when you would do something different. Veterinary assistants are generally not the equivalent of a formally educated veterinary technician and generally require much more supervision than a credentialed veterinary technician
There are voluntary educational opportunities, however these are not equivalent to a college degree program and are instead basic vocational training. There is no over-sight by a professional body to ensure that the majority of these programs provide adequate or correct information. There is no requirement for hands-on training and instructors often have little or no experience or education in the veterinary field. There are a handful of certification programs that are designed and approved by veterinary professional organizations or that are offered by colleges which also offer accredited veterinary technology programs and these are better choices for someone who wants to be a veterinary assistant. All of these programs offer certification as a veterinary assistant, but this certification is not legally recognized or required by any state in the US.
Veterinary technicians are required (in most states) to have a 2 year degree in veterinary technology from an AVMA accredited veterinary technology program, to have passed the Veterinary Technician National Exam and a state exam in order to be credentialed. They are also generally required to attend a set number of continuing education courses each year to keep up with changes in veterinary medicine. Veterinary technicians are educated in veterinary anatomy and physiology, pharmacology, animal husbandry, surgical assisting, anesthesia, medical nursing, diagnostics such as radiology and ultrasonography, clinical pathology, parasitology, medical terminology and record keeping, biological collection and sample handling and preperation, etc. They can also specialize in areas such as emergency and critical care, internal medicine, anesthesia, dentistry, behavior and equine nursing.
The American Veterinary Medical Association maintains a list of accredited degree programs on their website: http://www.avma.org/education/cvea/vettech_programs/vettech_programs.asp
In some states, the use of the title "veterinary technician" and the practice of veterinary technology is recognized as profession and licensure is required. In other states, veterinary technicians are registered or certified. The laws that govern veterinary technicians vary from state to state so for specific information on the laws a person should check their state veterinary practice act or contact their state veterinary licensing board.
Veterinary assistants tend to make close to minimum wage. Veterinary technicians will generally make more due to their education and increased abilities, but it is still not a great paying job. To find out numbers for your state, you should contact your state veterinary technician association or veterinary medical association as pay scales vary from state to state.
I've worked in veterinary clinics for 20+ years now and love the work. I started out as a veterinary assistant and then learned about the degree available in veterinary technology and went back to college. It can be very stressful and some days are disheartening, but there are also the great victories when a patient recovers. Pay and benefits tend to be low overall, but if you look around and pick positions at larger clinics or specialty practices the income tends to be higher.
Before enrolling in a veterinary technology program, it is a good idea to volunteer or take a job at a veterinary hospital to see what the job of a veterinary technician really entails. Many people think that it will suit them but find out differently once they start school. Having personal experience in a veterinary facility will also help you to excel in your classes by giving you real-world application for what you are learning.