What is the difference between a claims-made policy and an occurrence policy?

Insurance coverage can either be on a "claims made" or "occurrence" basis. Simply, these terms refer to what the policy covers during the policy period. A claims-made policy provides insurance for claims made during a policy year (or tail period) for losses that occurred after a specified retroactive (“retro”) date, and an occurrence policy provides insurance for losses that occurred only during the policy year (as specified on the declarations form) even if the claims arising out of those occurrences are made after the policy period. 

A "claims made" policy provides coverage for claims made (and reported) during the policy term as long as they arose out of occurrences that happened after the "retroactive date." (The retroactive date is the start date of the first claims-made policy obtained, so long as it has been kept active (via renewal).) That date remains the same and is not advanced as long as the policy is renewed. For example, if a client first obtains claims-made coverage on October 1, 2013, and it is now 2019 and they've maintained the same coverage, their claims-made policy covers all incidents since October 1, 2013. If the client no longer has coverage in 2020, and a patient sues in 2020 for an incident that occurred in 2013, the client does not have coverage for that incident (unless they purchased tail coverage). 

An "occurrence" policy, in contrast, covers incidents that occur (but do not necessarily give rise to claims) during the term the policy is in force. For example, if a client obtained an occurrence policy covering 2016-2017, and a patient is injured by the client's product in 2017, but does not sue for their injuries until 2019, the client has coverage from their 2016-2017 policy, even if they are no longer insured.