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.