When planning a budget you must first identify all the expenses you have. There are two general types of expenses - fixed & variable.
Fixed expenses are those that do not change. They happen every month, or sometimes multiple times per month, on a certain day, for a specific amount. These are things like mortgage/rent, car payment, cell phone bill, day care bill, utilities, and Netflix. When budgeting, these are the easiest types of expenses to plan for. You know when they will occur and how much you need to pay ahead of time.
Variable expenses are those that have some sort of change or variable. There are 3 types of variable expenses - non-recurring (aka: intermittent or periodic), recurring (aka: day-to-day or flexible) & whammy expenses.
Non-recurring expenses happen on a non-standard time table and include both bills such as annual subscriptions and tax payments, as well as semi-annual purchases like clothes, shoes, and school supplies. Similarly to fixed expenses, non-recurring expenses are somewhat easy to budget for because you usually know how much you need to pay. You can budget for these types of expenses by saving a little bit each month, that way when the bill comes, you have the money already saved up.
Recurring expenses are those that happen reliably but do not have predictable dates or amounts. Examples of recurring expenses include things such as gas, groceries, and dining out. When budgeting, these expenses can be planned for by examining prior spending. These categories will be somewhat fluid from month to month, depending on actual spending.
Aptly named, whammy expenses are just that, a whammy to your budget. These are the unpredictable expenses you cannot budget specifically for. Whammies include unexpected car and home repair and emergency medical or veterinary bills. You can never predict when these unfortunate issues will arise, but you can plan to budget for them. Putting some money aside in an emergency fund will help ease the damage of these types of expenses when they arise.