When you do your taxes, you have two choices for your deductions. You can take the standard deduction, or you can choose to itemize your deductions. If you itemize your deductions, you use Form 1040 Schedule A.
By looking at Schedule A, you can see the list of deductions that are itemized:
- Medical expenses
- Taxes you paid (state taxes, property taxes, other taxes)
- Interest you paid (home mortgage interest, investment interest)
- Gifts to charity
- Casualty and theft losses
- Job expenses and certain miscellaneous deductions
- Other miscellaneous deductions
On Schedule A itself, you only list a total for each of these broad categories. In some cases, this is sufficient detail. However, for certain deductions, finer detail may be required, and you may have to submit additional forms showing this detail. For example, on the medical expense line, you generally only list a total of medical expenses; details are only supplied to the IRS upon request. For noncash gifts to charity, you need to supply more details on Form 8283 if your gifts are worth more than $500.
These requirements can be found in the instructions for Schedule A.
As noted by @Accumulation in the comments, the above deductions that are a part of your itemized deductions are called "below the line" deductions (because they are subtracted after the adjusted gross income line) and are only able to be deducted if you choose to decline the standard deduction. There are other deductions that are available whether or not you itemize. These "above the line" deductions are found on Form 1040 Lines 23-35. If you look at these lines on the form, you'll see the different types of deductions that are called out here. Some of these deductions require additional details on other forms; for example, the HSA deduction requires details on Form 8889.
If you have a business, your business expenses are not part of your itemized deductions at all, and do not appear on Schedule A anywhere. Instead, your business expenses get subtracted from your business's revenue, and the resulting profit (or loss) is what is reported on your Form 1040.
Different types of businesses report these expenses differently. If you have a sole proprietorship, the details of your business's expenses are reported on Schedule C. On this schedule, Part II is devoted to deductible business expenses. Take a look at Schedule C, and you'll see that Lines 8-27 are different categories of expenses that get called out on this schedule.
