All values in Lua are first-class values. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results.

There are eight basic types in Lua: nil, boolean, number, string, function, userdata, thread, and table. Nil is the type of the value nil, whose main property is to be different from any other value; it usually represents the absence of a useful value. Boolean is the type of the values false and true. Both nil and false make a condition false; any other value makes it true. Number represents real (double-precision floating-point) numbers. (It is easy to build Lua interpreters that use other internal representations for numbers, such as single-precision float or long integers; see file .) String represents arrays of characters. Lua is 8-bit clean: strings can contain any 8-bit character, including embedded zeros (‘\0‘) (see ).

The type userdata is provided to allow arbitrary C data to be stored in Lua variables. This type corresponds to a block of raw memory and has no pre-defined operations in Lua, except assignment and identity test. However, by using metatables, the programmer can define operations for userdata values (see §2.8). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program.

The type thread represents independent threads of execution and it is used to implement coroutines (see ). Do not confuse Lua threads with operating-system threads. Lua supports coroutines on all systems, even those that do not support threads.

Like indices, the value of a table field can be of any type (except nil). In particular, because functions are first-class values, table fields can contain functions. Thus tables can also carry methods (see §2.5.9).

Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.

Lua provides automatic conversion between string and number values at run time. Any arithmetic operation applied to a string tries to convert this string to a number, following the usual conversion rules. Conversely, whenever a number is used where a string is expected, the number is converted to a string, in a reasonable format. For complete control over how numbers are converted to strings, use the format function from the string library (see ).