Menswear is typically broken down into different groups, identified by names such as formal business, business casual and casual. Casual menswear can include clothes from the former two groups.
Business casual includes khaki pants, polo shirts, buttoned-down shirts, sweaters, blazer and loafers. Casual clothing includes all the items from the business casual category as well as nice denim jeans, nice T-shirts and sneakers.
Casual menswear is often appropriate for any informal events, including casual Fridays at some places of work. When in doubt, select a mix between the two groups, such as a buttoned-down shirt, blazer, jeans and dark sneakers.
Ratty, torn or dirty jeans and T-shirts are usually not appropriate casual menswear. Tank tops, shorts, sandals and shirts with writing are also inappropriate for most situations.