Explain that no struct can be created as transaction input other than…

… some exceptions (#91)

book/src/concepts/what-is-a-transaction.md

@@ -29,6 +29,24 @@ Transactions consist of:
 - a gas object - the `Coin` object used to pay for the transaction;
 - gas price and budget - the cost of the transaction;
 
+## Inputs
+
+Transaction inputs are the arguments for the transaction and are split between 2 types:
+- Pure arguments: These are mostly [primitive types](../move-basics/primitive-types.html) with some
+extra additions. A pure argument can be:
+    - [`bool`](../move-basics/primitive-types.html#booleans).
+    - [integer](../move-basics/primitive-types.html#integers) (`u8`, `u16`, `u32`, `u64`, `u128`, `u256`).
+    - [`address`](../move-basics/address.html).
+    - [`std::string::String`](../move-basics/string.html), UTF8 strings.
+    - [`std::ascii::String`](../move-basics/string.html#ascii-strings), ASCII strings.
+    - [`vector<T>`](../move-basics/vector.html), where `T` is a pure type.
+    - [`std::option::Option<T>`](../move-basics/option.html), where `T` is a pure type.
+    - [`std::object::ID`](../storage/uid-and-id.html), typically points to an object. See also [What is an Object](../object/object-model.html).
+- Object arguments: These are objects or references of objects that the transaction will access. An
+object argument needs to be either a shared object, a frozen object, or an object that the
+transaction sender owns, in order for the transaction to be successfull.
+For more see [Object Model](../object/index.html).
+
 ## Commands
 
 Sui transactions may consist of multiple commands. Each command is a single built-in command (like