cells in UICollectionview can be of same Size or different sizes, so we use the flow layout. UICollectionViewFlowLayout manage items into a grid with header and footer view for every section.

Here we define the flow layout--

-(void)viewDidLoad
    {
        [super viewDidLoad];
        [self.collectionView registerClass:[CellClass class] forCellWithReuseIdentifier:@"identifier"];
        self.collectionView.backgroundColor = [UIColor clearColor];

        // Configure flow layout
        self.flowLayout = [[UICollectionViewFlowLayout alloc] init];
        [self.flowLayout setItemSize:CGSizeMake(width, height)];
        [self.flowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
        self.flowLayout.minimumInteritemSpacing = 0.0f;
        [self.collectionView setCollectionViewLayout:self.flowLayout];
        self.collectionView.bounces = YES;
        [self.collectionView setShowsHorizontalScrollIndicator:NO];
        [self.collectionView setShowsVerticalScrollIndicator:NO];
    }

Here flow layout is the property of UICollectionviewFlowLayout. The method below sets the spacing between sections within the collection view.

- (UIEdgeInsets)collectionView:
(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section {
    return UIEdgeInsetsMake(0, 0, 0, 0);
}