struct
a collection of types, composite data type, composition, “user-definded data structure”
struct Test {
char* name;
int nr;
float f;
};

Name of type is “struct Test”, and not “Test” alone.
struct Test myNewTest

Semicolon after closing parethesis is essentional.

Access composed types with dot-syntax:
myNewTest.name = "hjkjnk";
myNewTest.nr = 32;
myNewTest.f = 2.2424;

typedef
a shorthand, an alias for a type, for ease of readability.

typedef int MyType

“MyType n;” would be equal to “int n;”

enum
“introduces a symbolic name for an integer constant”

enum direction_enum {LEFT, RIGHT};
/* LEFT -> 0, RIGHT -> 1; */
enum direction_enum x = LEFT;

typedef enum

typedef enum {
LEFT;
RIGHT;
} direction;

declatation:
-(void)myFunction:(direction)LEFTorRIGHT;

call:
-(void)myFunction:LEFT;

typedef struct

typedef struct objectInfo_ {
CGPoint origin;
NSString* name;
NSColor* color;
} objectInfo

Shortcut. Instead of writing ‘struct objectInfo_’, write ‘objectInfo’. The keyword ‘objectInfo_’ seems to be optional.