I was playing with dd "conv=noerror" option. It continues reading the input file(for example HDD/SSD partition) even in case there are read errors. Which errors are critical for integrity of ext[234] file system? As I understand, Linux views the file system as a common set of objects- superblock(maintains and describes the state of the file system), inode(unique identifier for all the file system objects and contains necessary metadata), dentry(glues together inode number and file name), and file(bunch of bytes arranged in a certain order). Am I correct that if the main superblock is damaged, the spare one is used by fsck? In addition, am I correct that if sectors which hold content of a file(for example .avi movie) are damaged, then only this file is corrupted but file system itself is in good shape?

Your assumptions are correct.