An archiver (or "archival utility") is a program which combines many files into a single file, with later means for getting the files out. This can be done for a number of purposes; for example, just to stick the files together (as with "tar"), to convert the files so that they can pass through some limited form of communication such as email ("shar", AKA "shell archive") or even to also compress the files for minimum file sizes ("zip", "rar", etc.)
When we want to send files over the internet, we typically want to compress these files so that they take up as little space as possible. We generally also need to bundle files together. Today, the default format for doing this on personal computers is the PKZIP format, while on Unix systems we use tar to group files together, and a separate program to compress them - these days that's generally gzip or bzip2, and in olden times it was the compress program. A lot of PC users are also using the rar archiver, because for a long time it has provided the best compression.
Today, that is no longer true; the crown is worn by 7-Zip. 7-Zip is a newer archive manager which, in addition to being able to handle older archive formats at various levels (for example, it can decompress but not create rar archives, but it can both make and break zip archives) it implements the new 7z compression algorithm. This is not actually the most efficient algorithm available today, but it is the best one that has a decent application wrapped around it.