Insurance companies may be classified into two groups:
- Life insurance companies, which sell life insurance, annuities and pensions products.
- Non-life, general, or property/casualty insurance companies, which sell other types of insurance.
General insurance companies can be further divided into these sub categories.
- Standard lines
- Excess lines
In most countries, life and non-life insurers are subject to different regulatory regimes and different tax and accounting  rules. The main reason for the distinction between the two types of  company is that life, annuity, and pension business is very long-term in  nature — coverage for life assurance or a pension can cover risks over  many decades. By contrast, non-life insurance cover usually covers a shorter period, such as one year.
In the United States, standard line insurance companies are  "mainstream" insurers. These are the companies that typically insure  autos, homes or businesses. They use pattern or "cookie-cutter" policies  without variation from one person to the next. They usually have lower  premiums than excess lines and can sell directly to individuals. They  are regulated by state laws that can restrict the amount they can charge  for insurance policies.
Excess line insurance companies (also known as Excess and Surplus)  typically insure risks not covered by the standard lines market. They  are broadly referred as being all insurance placed with non-admitted  insurers. Non-admitted insurers are not licensed in the states where the  risks are located. These companies have more flexibility and can react  faster than standard insurance companies because they are not required  to file rates and forms as the "admitted" carriers do. However, they  still have substantial regulatory requirements placed upon them. State  laws generally require insurance placed with surplus line agents and  brokers not to be available through standard licensed insurers.
Insurance companies are generally classified as either mutual  or stock companies. Mutual companies are owned by the policyholders,  while stockholders (who may or may not own policies) own stock insurance  companies. Demutualization  of mutual insurers to form stock companies, as well as the formation of  a hybrid known as a mutual holding company, became common in some  countries, such as the United States, in the late 20th century.
Other possible forms for an insurance company include reciprocals, in which policyholders reciprocate in sharing risks, and Lloyd's organizations.
Insurance companies are rated by various agencies such as A. M. Best.  The ratings include the company's financial strength, which measures  its ability to pay claims. It also rates financial instruments issued by  the insurance company, such as bonds, notes, and securitization  products.
Reinsurance  companies are insurance companies that sell policies to other insurance  companies, allowing them to reduce their risks and protect themselves  from very large losses. The reinsurance market is dominated by a few  very large companies, with huge reserves. A reinsurer may also be a  direct writer of insurance risks as well.
Captive insurance  companies may be defined as limited-purpose insurance companies  established with the specific objective of financing risks emanating  from their parent group or groups. This definition can sometimes be  extended to include some of the risks of the parent company's customers.  In short, it is an in-house self-insurance vehicle. Captives may take  the form of a "pure" entity (which is a 100% subsidiary of the  self-insured parent company); of a "mutual" captive (which insures the  collective risks of members of an industry); and of an "association"  captive (which self-insures individual risks of the members of a  professional, commercial or industrial association). Captives represent  commercial, economic and tax advantages to their sponsors because of the  reductions in costs they help create and for the ease of insurance risk  management and the flexibility for cash flows they generate.  Additionally, they may provide coverage of risks which is neither  available nor offered in the traditional insurance market at reasonable  prices.
The types of risk that a captive can underwrite for their parents  include property damage, public and product liability, professional  indemnity, employee benefits, employers' liability, motor and medical  aid expenses. The captive's exposure to such risks may be limited by the  use of reinsurance.
Captives are becoming an increasingly important component of the risk  management and risk financing strategy of their parent. This can be  understood against the following background:
- heavy and increasing premium costs in almost every line of coverage;
- difficulties in insuring certain types of fortuitous risk;
- differential coverage standards in various parts of the world;
- rating structures which reflect market trends rather than individual loss experience;
- insufficient credit for deductibles and/or loss control efforts.
There are also companies known as 'insurance consultants'. Like a  mortgage broker, these companies are paid a fee by the customer to shop  around for the best insurance policy amongst many companies. Similar to  an insurance consultant, an 'insurance broker' also shops around for the  best insurance policy amongst many companies. However, with insurance  brokers, the fee is usually paid in the form of commission from the  insurer that is selected rather than directly from the client.
Neither insurance consultants nor insurance brokers are insurance  companies and no risks are transferred to them in insurance  transactions. Third party administrators are companies that perform  underwriting and sometimes claims handling services for insurance  companies. These companies often have special expertise that the  insurance companies do not have.
The financial stability and strength of an insurance company should  be a major consideration when buying an insurance contract. An insurance  premium paid currently provides coverage for losses that might arise  many years in the future. For that reason, the viability of the  insurance carrier is very important. In recent years, a number of  insurance companies have become insolvent, leaving their policyholders  with no coverage (or coverage only from a government-backed insurance  pool or other arrangement with less attractive payouts for losses). A  number of independent rating agencies provide information and rate the  financial viability of insurance companies.