How is technical writing and general writing different?

Technical writing is usually focused on technology related topics. It is not intended for a general audience, and its purpose is practical utility rather than entertainment. No one reads the assembly instructions for furniture or coding standards for military software for fun. Instead, technical writing addresses the needs of people using or developing specific devices.
Whether one is writing documents for users, developers, contractors, or one's own project team, technical documentation must be extremely accurate and complete in its coverage. Your camera manual needs to cover every ability of your camera, not just "five fun facts" about the camera. 
Technical writing is also unusual in needing regular updates. While, for example, the original Romeo and Juliet is still worth reading, an obsolete technical manual is useless.  Because technical manuals are frequently updated, they usually have numbered sections and change logs so that they can document changes in the products and procedures they describe.