There are four different types of Products:
Good - Physical items that may nor may not require stock quantity. Examples include things like a chalk bag, or climbing shoes.
Service - A membership item, that membership plans are attached to. For example you could have a Monthly Membership service product, with a $20 Monthly Plan, and a $30 Monthly Plan attached to it. Service products must be synced with Stripe.
Gift Card - A type of coupon that a member can purchase, which can be redeemed by another member.
Punch Card - A temporary membership based on a maximum number of allowed visits.
A product setup as a Good, can have optional variants as part of it. For example you may sell T-Shirts, in multiple sizes, which have different price points. Once a product has variants setup, a new field will become available when the product is selected when creating an invoice.
When creating a product you have a number of options to configure.
Title - The title of the product.
Description - The description of the product.
Product Type - The type of product, based on the (4) outlined above.
Price - The price in dollars and cents.
Quantity - The current quantity on hand for in stock items.
In Stock - Is the item in stock?
Tax Exempt - Set the product to be exempt from being taxed.
Product Image - An image for the product.
Variants - A variation of pricing as outlined in the variants above.
Stripe Sync - Should this product be synced to Stripe?