RDBMS: Proper way to store data - comma separated variables or in different fields or tables?

Fundamental question that I cannot seem to find an answer on. I have a database that stores line items from receipts along with the username and receipt number. Right now the lineItem column is just a long string of data separated by commas (original file appears to have been just an Excel file). This information is parsed in a PHP script for viewing on the front end. The table looks like this: |----------|----------|----------| |lineItem |receiptID |customerID| |----------|----------|----------| |CD, DVD, |001 |User01 | |----------|----------|----------| |CD, CD, |002 |User02 | |DVD, usb, | | | |----------|----------|----------| Ultimately, **is this bad practice**? Should the lineItem values be linked to related values in another table instead maybe?