Software that is available freely at no cost for download or usage. It should be devoid of any adware, spyware or any malware. It also should no be crippled of all functions.
Irfanview is real freeware.
Similar to shareware, freeware is software that is distributed with no requirement for a license or under the GPL (GNU Public License) - that is, legally free software.
Note that this is different than warez; free versions of commercial software.
Windows XP is commercial software, but most versions of Linux are freeware.