True Buddhism is the term used for itself by some Buddhists who feel their teachings are "orthodox" and those of rivals are not orthodox. It is often a misnomer because Buddhism has come to be a family of concepts and teachings some of which directly contradict each other. Moreover most concepts in Buddhism are inextricably linked to the religious ideas of the host countries where Buddhism once was practiced. The Sokagakkai and their founder, Nichiren, the "True Buddha" calls itself "True Buddhism" but like most such groups other Buddhists don't accept that idea.
However, as a pure concept it is worth aiming for, because there is a common thread that marks "true Buddhist" ideas, and that can be found across the board. True Buddhism is Buddhism that tries to be true to its roots, both by walking in the footsteps of the founder Shakyamuni, and in the footsteps of those who succeeded him. The common thread here is an emphasis on clear thinking, enlightenment, and a willingness to accept reality for what it is rather than what folks would like it to be. That entails following logic, but not at the expense of "heart". I feel that true Buddhism marks more an approach to religion than any particular brand of Buddhism or for that matter any particular host religion.
When Buddhists go into an area with the goal of spreading enlightenment and straitening up what is "twisted" "shakubuku", they can accomplish great things. When they set out to create a religion that displaces and replaces those of the host nation they almost always fail. A better approach for Buddhists is to examine the ideas of the host country and accommodate those that can be accommodated while critiquing or changing those that are false, convenient, or self-destructive. In my opinion this means that Buddhism should not see itself as an utterly separate religion outside host countries that are already Buddhist but as a group aimed at improving the life and enlightening people side by side with those host religions that behave in an accepting manner. Many Buddhists disagree with that opinion. But it is mine. The principle of accommodating local custom is known as Zuiho Bini.