A constitutional monarchy is a form of government in which a monarch acts as head of state within the perimeters of a written (i.e., codified), unwritten (i.e., uncodified) or blended constitution. It differs from absolute monarchy in that an absolute monarch serves as the sole source of political power in the state and is not legally bound by any constitution.
Most constitutional monarchies employ a parliamentary system in which the Monarch may have strictly Ceremonial duties or may have Reserve Powers, depending on the constitution, have a directly or indirectly elected prime minister who is the head of government and exercises effective political power. In the past, constitutional monarchs have co-existed with fascist and quasi-fascist constitutions (Fascist Italy, Francoist Spain) and with military dictatorships.
Contemporary constitutional monarchies include Australia, Bahrain, Belgium, Belize, Cambodia, Canada, Denmark, Japan, Jordan, Lesotho, Luxembourg, Malaysia, Morocco, New Zealand, Netherlands, Norway, Spain, Sweden, Thailand, United Kingdom, and Vatican City.
A republic is a form of government in which the citizens choose their leaders  and the people (or at least a part of its people) have an impact on its government. The word “republic” is derived from the Latin phrase res publica, which can be translated as “a public affair”.
Both modern and ancient republics vary widely in their ideology and composition. The most common definition of a republic is a state without a monarch. In republics such as the United States and France the executive is legitimated both by a constitution and by popular suffrage. In the United States, James Madison defined republic in terms of representative democracy as opposed to direct democracy, and this usage is still employed by many viewing themselves as “republicans”. In modern political science, republicanism refers to a specific ideology that is based on civic virtue and is considered distinct from ideologies such as liberalism.
Most often a republic is a sovereign country, but there are also subnational entities that are referred to as republics, or which have governments that are described as “republican” in nature. For instance, Article IV of the Constitution of the United States “guarantee[s] to every State in this Union a Republican form of Government.” The Soviet Union was a single nation composed of distinct and legally sovereign Soviet Socialist Republics.
Just my guess…
Thailand is a constitutional Monarchy which is a parliment with a prime minister and a king. The republic I guess could be like the US which only has a president or something like Russia which has a parliment, Prime minister, and president.