Input split is the logical representation of Block size. One split is usually equal to one block. But block size and split size can be customized. InputFormat.getSplitts() is responsible for creating the splits which are used as input my Mapper. One mapper can process one split at a time.
Consider block size of 128 Mb. Suppose a large file comes and is broken into blocks. One block can end in between a line but the split will consider this and it ends at the completion of the previous line.