The certifications
Tableau Desktop Specialist is the entry-level exam: foundational concepts, no expiry. Tableau Certified Data Analyst is the professional-level Desktop credential, covering preparation, exploration, analysis, and sharing. On the server side, Server Associate and Server Certified Professional cover administration.
Exact names, pricing and durations change periodically, so confirm the current lineup on Tableau's certification site before booking.
What gets tested most
Across the Desktop exams, weight concentrates on: connecting and preparing data (joins, blending, extracts), exploring and analysing (calculations, LODs, table calculations, filters), and sharing (dashboards, actions, publishing).
The order of operations and the difference between LOD types come up repeatedly, in both knowledge and hands-on form. So does knowing which chart suits which question.
How to study
Hands-on questions are performed in a live Tableau environment, so reading alone won't get you there. Build things.
A workable plan: work through the lessons in each track here, then rebuild each concept from scratch in Tableau with the Superstore sample data. If you can construct a FIXED LOD or a running total without looking it up, you know it. Use the quizzes for recall and the flashcards for the last day before the exam.
Time yourself. Running out of time is a more common failure than not knowing the material.