A journal publishes research articles and other written pieces (such as opinion pieces and book reviews). Many journals publish collections of these pieces called issues every month, quarter, or year. Some journals publish articles individually throughout the year.
Journals normally follow a theme. For example, the Journal of Nutrition publishes articles related to nutrition. The journal Science publishes pieces related to science. You can find out more about a journal's theme by looking for information about its scope on its website. Usually, a journal is run by a group of scholars whose work relates to that theme. They are called the editorial board.
Many journals use peer review, but some journals do not. For more about peer review, see "What does 'peer-reviewed' mean?" or "How do I find peer-reviewed articles?"