Useful terminology

Operators are the basic building blocks. You can combine them into graphs.
Symbols are definitions of operators. If you update these definitions, all their Instances will automatically update as well (aka Templates or Blueprints). Internally, symbols are C# classes that are compiled to run very fast.
Instances are actual copies of Symbols. Imagine a Symbol being a stamp that creates a copy. Instances are where your data lives. E.g. there could be two instances of [LoadImage], one loading a frog and another loading a toad.
Graph — a Symbol can contain many children that reference other symbols. These children are connected into a network. The Symbol then exposes inputs and outputs that let you connect its children to its outside. See [HowTixlWorks] for more details.
Projects (aka Packages) define a set of Symbols that can be reused, like your personal projects. TiXL comes with a number of built-in, read-only projects like Lib and Examples.