SSS = side side side
It's used only when you know 3 pairs of congruent sides. Tickmarks are often used on diagrams to indicate which pairs of sides match up.
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SAS = side angle side
This is when we know two pairs of congruent sides, and also a pair of congruent angles. The angles must be between the congruent sides. Otherwise you'll have SSA which is not a valid congruence theorem.
Congruent angles will have similar tickmarks on them. Use other theorems such as the alternate interior angle theorem to help pair up congruent angles.