What QIF is
QIF is a plain text list of transactions, one field per line, each line beginning with a single letter that says which field it is. D is the date, T the amount, P the payee. A caret on its own line ends a transaction.
Quicken, and the long tail of banks and budgeting apps that still offer it because everything can read it.
What Excel is
An Excel workbook is a zip archive of XML parts describing sheets, cells, formulas, styles and charts. The .xlsx extension is that format; .xls is the older binary one from before 2007.
Finance, operations and anyone who received a file from finance or operations.
What changes when you convert QIF to Excel
Each block up to a caret becomes a row. Split transactions, which QIF writes as repeated S, E and $ lines, are joined into single cells rather than being spread across as many columns as the most-split transaction in the file has parts. The account type from the !Type header is added as a column.
You get a single sheet with a header row and column widths sized to the content. Values are written as text unless type inference is on, which keeps identifiers like 0044 or 007 intact.
Each block ending in a caret becomes a row. The single-letter codes become named columns: D is the date, T the amount, P the payee, M the memo, L the category, N the cheque number.
Split transactions are joined rather than spread out. QIF writes a split as repeated S, E and $ lines, and giving each part its own column would make the table as wide as the most-split transaction in the file while leaving every simple transaction mostly empty.
The day and month order is left exactly as the file wrote it. QIF has no field saying which way round its dates are, so a file from a US bank and one from a European bank look identical and mean different things. Check one date you know before you rely on the rest.
What carries over from QIF to Excel
One property of the QIF has no home in Excel, and it is worth knowing which before you convert.
Excel wants a type for each column, and QIF does not record one, so each column is typed from what its values look like. A column of digits that should stay text - a zip code, a phone number, a leading-zero id - is the usual thing to check afterwards.
QIF can be read one record at a time. Excel has to be complete before it means anything, so the whole table is held in memory while it is written and a file that gets cut off part way through is not partly usable - it is unusable.