Let's start with the basics, Strict constructionist and loose constructionist are two of several methods of interpreting the Constitution, and the most commonly referred to. Strict constructionist is a sole adherence to the words used in the Constitution and only to be interpreted as such. No other interpretation is needed other than what is provided in the written text. I will get to the latter later on.
A clear distinction must be made between “strict constructionist” and “original intent". “Original intent” or "Originalism" is a method of interpretation that considers the meaning of the words to the ratifying states at the time of the Constitution's ratification. In other words, how did the states interpret the meanings of the text in order to win their votes? After all, the only reason we have the Constitution today is because those states saw fit to vote for it and ratify it due to the meanings of the words at the time. Essentially, original intent was the only jurisprudence of the founders and framers of United States and Constitution respectively (obviously, since they established it on those very meanings, the reason for its existence today). Antonin Scalia and Clarence Thomas are examples of judges who currently employ this judicial philosophy. “Original intent” is not to presume that anyone knew what the framers were thinking about specifically when they drafted the law, which would be an impossible task. Moreover, that would most certainly imply a hidden meaning or an acknowledgement thereof, allowing manipulation and bias into the judicial process. The Framers could have been thinking about puppy dogs and ice cream when they wrote the first amendment or drafted Article I, but that is wholly irrelevant.
A loose constructionist (the jurisprudence of judicial activism) would view the constitution as a “living, breathing thing”, merely a guideline that can be interpreted by how an individual judge's view the words in the constitution. This idea, with no coincidence, crept in along with the Fabian socialist ideas of the progressive movement. Woodrow Wilson advocated for a more "political" court, he felt that the constitution should not be interpreted in its "strict letter" but rather in its "spirit". Wilson explains it as:
"That field they respectfully avoid, and confine themselves to the necessary conclusions drawn from written law. But it is true that their power is political; that if they had interpreted the Constitution in its strict letter, as some proposed, and not in its spirit, like the charter of a business corporation and not like the charter of a living government, the vehicle of a nation's life, it would have proved a strait-jacket, a means not of liberty and development, but of mere restriction and embarrassment."
What is a living government? Governments don't live, people do. Moreover, the Constitutions entire purpose was restriction of the federal government; this notion that natural law/liberty are conferred by government is insane. A once over of the Declaration of Independence is all a 2nd grader would need to understand that. The Court-as defined by the Constitution- is not at all to be political and was purposely made not to be subject to the whims of the political process. Hence, their lifetime appointments rather than elections and specific powers enumerated in the constitution. Checks and balances in government only work when each branch is "separate and distinct" (The words of James Madison, the "father" of the Constitution, federalist No. 48) Surely, this doesn't mean that nine unelected lawyers should have the power to usurp legislative duties from Congress. The first notable judicial activists were appointed by FDR, no coincidence there either. He appointed justices favorable to his massive overreach of government (after both NRA and AAA were declared unconstitutional) and a court that eventually gave us Korematsu and Filburn (Filburn destroyed the original intent of the commerce clause and basically instituted central planning/socialism). Judicial activism is a viewpoint and process from which the federal government can deviate from the pure meaning of the ratifiers and the text, on a case-by-case interpretation, in other words, convey the ideological predilections of the Justices’. This "living" and "breathing" form of jurisprudence implies that the Constitution can be manipulated to present popular sentiment, relation to foreign law or subject to one’s own ideology. The ease in which five lawyers (a majority of the nine justices) can manipulate the law to reflect their own vision is stunning and is oligarchical in nature. As we have seen with Roe, Korematsu, Plessy, kelo, Dread Scott, and more recently the DOMA and Obamacare rulings. The practice of Stare Decisis is a further impediment to overturning the unconstitutionality of most laws.
Judicial activism is conferring a judge’s own ideology on the words of the constitution in order to institute legislation without the consent or accountability of the body politic. This method is used to affect change in the law without the proper constitutional amendment process (by-passing the people). Essentially, this was a way that the statist could circumvent the amendment process and impose their ideology on the individual. One cannot discuss Judicial Activism without mentioning the singular activist Supreme Court decision that led to all others, Marbury V. Madison.
In Marbury the court usurped the power of judicial review and constitutional authority away from the States and legislature, and created its own power not enumerated in the Constitution (Article III, Section 1 and 2). The Marbury court ruling gave all power of judicial review solely to the federal government via the Judiciary. Not only did the Federal Government make laws, they also were the sole grantor of constitutional authority of those laws. It was the very first breach of separation of powers, not only between the branches of the Federal Government but the diffusion of the Federal Governments power amongst the States, as Thomas Jefferson so eloquently summarizes, "The constitution, on this hypothesis, is a mere thing of wax in the hands of the judiciary, which they may twist and shape into any form they please." This piecemeal approach to the judiciary by the Fabians led to the the soft tyranny we have today.