structure in objective C

A C structure is a compound data type: it combines multiple data types into a single type, which can be passed around as a single entity and can be accessed by those names through the compound entity, using dot-notation. The iOS API has many commonly used structs.
For example,  CGPoint is defined as follows:
struct CGPoint
{
     CGFloat x;
    CGFloat y;
};
 typedef struct CGPoint CGPoint;

So we can write:
    CGPoint myPoint;
    myPoint.x = 4.3;
    myPoint.y = 7.1;

Another iOS struct,
CGSize:
struct CGSize
{
    CGFloat width;
    CGFloat height;
};
 typedef struct CGSize CGSize;

Put a CGPoint and a CGSize together and you’ve got a CGRect:
struct CGRect
{
    CGPoint origin;
    CGSize size;
};
typedef struct CGRect CGRect;