The underlying rules of the Star Wars Saga Edition (SWSE) TTRPG should feel largely familiar to players used to Dungeons & Dragons or other similar d20 TTRPGs, with a few key differences. Listed below are some of the major differences between SWSE and a game like D&D 3.5e:
Characters in SWSE do not have an Armor Class (AC), nor do they have saving throws. Instead, these two game features are merged into a single set of three defense scores possessed by every character: Reflex (Ref), Fortitude (Fort), and Will. Rather than making saving throw rolls with these defenses, the three numbers instead function like three seperate armor classes. A character uses their Ref score as their armor class when being targeted by most ranged and melee attacks, their Fort score when targeted by bodily trauma, physical hardships, poisons, environmental conditions, etc., and their Will score when targeted by fear, emotional manipulation, and other sorts of mental attacks.
Ref is Dexterity-based, Fort is Constitution-based, and Will is Wisdom-based. Actions, equipment, and feats which would be familiar to D&D players as providing an AC bonus will instead usually provide a bonus to Ref and sometimes to Fort.
In addition to their three defense scores, character also have a Damage Threshold (DT) stat. A character's DT is nominally equal to their Fort score, and determines how much damage they can take from a single attack before suffering additional negative effects beyond just the loss of hit points.
In addition to their hit points (HP), a character's health is also measured by their position on the Condition Track. A character at the top of the condition track (step 0) is in a "normal state", and though they may not be at maximum HP they are nonetheless unimpeded by their injuries. A character moved down the condition track (steps -1 through -4) takes progressively higher penalties to their defense scores, attack rolls, skill checks, and movement speed. A character moved to the bottom of the condition track (step -5) is either unconscious or dead, depending on the exact circumstances.
When the damage from a single attack exceeds a character's DT, the character is automatically moved -1 step on the condition track. Other combat actions and environmental conditions can also move characters down the condition track. Depending on the cause, moving back up the condition track may be as simple as a character pausing a moment to catch their breath, while in other cases the thing that moved a character down the condition track might result in a 'persistent condition' that cannot be removed without some more involved response (such as 8 hours rest, surgery, etc.)
Talents are similar to feats, except that they are each tied to a specific character class. In my SWSE homebrew rules, every time a character gains a level they learn one feat (chosen from the complete list of all feats) and one talent (chosen from the list of talents available to their chosen character class for that level).
Characters do not spend skill points to gain skill modifiers one rank at a time, as is the case in D&D 3.5e. Instead, skills are primarily learned via the Skill Training feat, which grants a +5 modifier on checks with the chosen skill. Most skills have some uses that can be attempted by any character, and other uses that can only be attempted by a character trained (i.e. having taken the Skill Training feat) in that skill. Other feats can further increase a character's skill modifiers, such as the Skill Focus feat which grants a further +5 modifier to a chosen skill.
Aside from feats, the only other way a character's skill modifiers increase is when they get bonuses to their Class Skills as they gain levels. Characters get a +1 bonus to class skills for every 2 levels they possess in a given class. (This is a change from vanilla SWSE, where characters instead get a +1 bonus per 2 levels in all skills, not just their class skills.)
The 6-second combat round is divided up almost the same in SWSE as in D&D 3.5e. On their turn, a character can either take one full-round action, or else they can take one standard action, one move action, and one swift action. A character can take a swift action in place of a move action, or can take a swift action or move action in place of a standard action, and thus a turn may instead consist of a standard action and two swift actions, or two move actions and a swift action, or a move action and two swift actions, or three swift actions.
Characters can also take any number of free actions on their turn. A reaction is a free action that can be taken by a character even when it is not their own turn.