The base type for collections in VL is the Sequence. It corresponds to what is known as IEnumerable in .net world. We just gave it a more human-readable name.
The SpreadBuilder is the mutable variant of the Spread for use in scenarios where performance counts. A typical scenario would be when slices need to be added/removed to/from a spread in a loop. In such a case use a SpreadBuilder to modify the spread and use ToSpread in the end to go back into the save immutable world.
The Dictionary is a key-value collection. Items (values) are added to the dictionary given a label (key). Keys have to be unique and can therefore be used to retrieve individual values from the dictionary.
The key is often a string, but can really be any other datatype.