"The time for empty talk is over," he told the crowd assembled at the Capitol. "Now arrives the hour of action."
A number of those actions roll back former President Barack Obama's policies and agenda -- another promise that Trump made during the campaign.
On Friday, Jan. 27, Trump signed an executive order -- “Protecting the Nation From Foreign Terrorist Entry Into the United States" -- suspending the refugee program for 120 days and halting immigration from seven predominantly Muslim countries for 90 days.
Orders are subject to repeal or modification by Congress in certain cases and review by the Supreme Court.
Executive orders and presidential memorandums fall under the umbrella of executive action (as do proclamations). The two types of presidential actions are not quite the same thing, but their effective differences are slight, if any.
"The distinction between these instruments -- executive orders, presidential memoranda and proclamations -- seems to be more a matter of form than of substance, given that all three may be employed to direct and govern the actions of government officials and agencies," the Congressional Research Service says in a report.
According to Huq, an "executive order is an instruction in writing issued from the White House by the president that is usually directed at an agency or department within the government that ... when it jumps through certain procedural hoops, has the force of law," he told ABC News in an interview.
One difference between memorandums and orders is that the language may differ, experts say.
"You'll see that in any of the executive orders that Trump issues, there's going to be some phrase right in the beginning that cites either constitutional or statutory authority delegated from Congress," said Daniel Gitterman, a public policy professor at the University of North Carolina at Chapel Hill and the author of "Calling the Shots: The President, Executive Orders and Public Policy."
Two of Trump's executive orders start with "By the authority vested in me as president by the Constitution and the laws of the United States of America, it is hereby ordered as follows ..."
Executive orders are numbered. After a president signs an executive order, the White House sends the order to the Office of the Federal Register, which keeps a log of all executive orders. The OFR numbers each executive order sequentially.
Presidents may revoke, modify or replace their own orders and their predecessors', according to the Congressional Research Service.
On Inauguration Day, Trump put his signature on his first presidential memorandum. On Monday he signed three presidential memorandums, including one that withdraws the U.S. from the Trans-Pacific Partnership, and four more memorandums on Tuesday.
There is no formal procedure that defines what a presidential memorandum is, what it may do or how it may be issued, according to Huq.
"[Presidential memorandums] are used in the last 20 years much in the same way as an executive order gets used but without the level of formality and process and input from different officials in the administration that executive order requires," Huq said.
Gitterman believes that the Trump administration will likely use a mix of executive orders and memorandums.
Unlike executive orders, memorandums aren't thoroughly recorded by the government.
"Memorandums go below the radar much more and are harder for, I think, the news media and the public to track," Gitterman said.
"It's kind of a problem that we don't have any stable framework for this," Huq said of keeping track of presidential directives. "The way it's done seems to change over time along with the stout necessities of the moment, and that seems very ad hoc."
Trump's White House appears to be tracking and publishing executive orders, memorandums and proclamations on its website (although it may not be up to date).
"As a matter or historical practice, however, it seems that presidents are more apt to utilize executive orders on matters that may benefit from public awareness or be subject to heightened scrutiny," the Congressional Research Service report says. "Memoranda, on the other hand, are often used to carry out routine executive decisions and determinations or to direct agencies to perform duties consistent with the law or implement laws that are presidential priorities."
What They Do and Don't Do
According to the Congressional Research Service, executive orders "if they are based on appropriate authority ... have the force and effect of law."
"While the president's ability to use executive orders as a means of implementing presidential power has been established as a matter of law and practice, it is equally well established that the substance of an executive order, including any requirements or prohibitions, may have the force and effect of law only if the presidential action is based on power vested in the president by the U.S. Constitution or delegated to the president by Congress," the CRS says. "The president's authority to issue executive orders does not include a grant of power to implement policy decisions that are not otherwise authorized by law."
The report cites a widely accepted 1957 interpretation from the House Government Operations Committee that says executive orders "usually affect private individuals only indirectly" while proclamations "in most instances affect primarily the activities of private individuals."
The Supreme Court has ruled on the validity of executive orders, and the standard appears to be a 1952 case involving President Harry Truman, the CRS report says. The majority in that case ruled that the president, as executor of the laws, may not make law, deeming an action by Truman unconstitutional.
A concurring opinion in that case by Justice Robert Jackson appears to have held the most weight, describing three cases for presidential action: the strongest when the chief executive is expressly authorized by Congress or the Constitution to do something, the next tier for situations in which Congress and the Constitution are silent and the lowest tier for going directly against the will of Congress, the CRS says in its report.
In addition to successive presidents, Congress may modify or nullify orders that are based on authority granted to the president by Congress, according to the CRS report, but the legislature has rarely done so in modern times.