A journal entry is the recording of an event posted into a journal.  Journal entries are typically made into the company's general journal.  Journal entries must always have an equal amount of debits and credits, to balance the entry.  All journal entries must also contain a date and description.

Journal entries typically record manual transactions outside of your checkbook, invoicing, and payables modules.  They often post non-cash transactions into the general ledger.  

Common journal entries include recording depreciation, accruing interest or expenses, or adjusting entries provided by your accountant.

For more information about creating journal entries, see the article How to create a journal entry

For more information on debits and credits, see the article What are debits and credits?