The third five consonants -- ट, ठ, ड, ढ, ण -- are cerebral consonants ( Cerebral consonants). Apparently, in most phonetic systems, these are known as retroflex consonants, but in this and other languages in the region they are called cerebral consonants. From my point of view, as an American, cerebral consonants require tongue placement similar to the palatal consonants. In palatal consonants, such as /j/, the flat part fo the tongue presses against the hard palate on the roof of your mouth. In cerebral consonants, from this position in articulating a palatal consonant, the tip of the tongue touches the roof of the mouth behind the point where palatal consonants are articulated. (I may be wrong -- comments are welcome.)

The cerebral and dental d's and t's take some getting used to. I grew up with one d and one t in written English and no distinction between any of the different d's and t's in spoken English. But, in Hindi, there are four each: unaspirated cerebral; aspirated cerebral; unaspirated dental; aspirated dental.

ट
ṭa, /ʈ/

ठ
ṭha, /ʈ^h/

ट is said with no aspiration, ठ is said with aspiration.

ड
ḍa, /ɖ/

ढ
ḍha, /ɖ^h/

ड is said with no aspiration, ढ is said with aspiration.

ण
ṇa, /ɳ/

ण is the first common nasal consonant, i.e., you'll tend to see this one more than the previous two (ङ and ञ).

I am archiving this information about the alphabet on the Hindi page: kirkkittell.com/language/hindi. More information from Wikipedia:

• Cerebral consonants
• Sounds represented within /.../, e.g., /k/, are symbols of the International Phonetic Alphabet (IPA), which is extremely useful for me to understand how Hindi sounds.
• Sounds such as ka are transliterations from the International Alphabet of Sanskrit Transliteration (IAST). It's simpler to understand than IPA.