Ablution is one’s purification with water from minor impurity.
Tayammum is purification with earth (soil), from minor and major impurity in case of the inability to use water.
(But if you are ill, on a journey, or have relieved yourselves, or been intimate with your wives and cannot find water, then purify yourselves with clean earth, wiping your faces and hands. And Allah is Ever-Pardoning, All-Forgiving.) An-Nisa 4: 43
التيمم طهارة بالتراب من الحدث الأصغر و الأكبر في حال عدم القدرة على استعمال الماء
﴿المائدة: ٦﴾
-وَإِن كُنتُم مَّرْضَىٰ أَوْ عَلَىٰ سَفَرٍ أَوْ جَاءَ أَحَدٌ مِّنكُم مِّنَ الْغَائِطِ أَوْ لَامَسْتُمُ النِّسَاءَ فَلَمْ تَجِدُوا مَاءً فَتَيَمَّمُوا صَعِيدًا طَيِّبًا فَامْسَحُوا بِوُجُوهِكُمْ وَأَيْدِيكُمْ إِنَّ اللَّـهَ كَانَ عَفُوًّا غَفُورًا-