The main difference
TOPIX is the broader Japan stock-market index. It covers a wide group of listed Japanese companies and is often used when investors want broad domestic equity exposure.
Nikkei 225 is narrower. It tracks 225 selected Japanese companies and is the headline number many people see in market news.
Simple example
The TOPIX ETF page groups broad Japan-market listings. ETF Note currently lists 42 TOPIX-related ETFs.
The Nikkei 225 ETF page groups ETFs tied to the well-known 225-stock benchmark. ETF Note currently lists 28 Nikkei 225-related ETFs.
ETFs to compare
For TOPIX, examples include 1308 Listed Index Fund TOPIX, 1348 MAXIS TOPIX ETF, 1473 One ETF TOPIX, 2557 SMDAM TOPIX ETF. For Nikkei 225, examples include 1330 Listed Index Fund 225, 1346 MAXIS NIKKEI225 ETF, 1578 Listed Index Fund Nikkei 225 (Mini), 1369 One ETF Nikkei225.
Compare the fee, issuer, trading unit, market-maker flag, and fund type. The same category can include normal ETFs, leveraged ETFs, and inverse products.
How the categories differ
TOPIX is the broader category because it represents more of the Japanese equity market.
Nikkei 225 is the more familiar headline index, but it is not the same as owning the whole Japanese market.