Nominal words include words like:
nouns, which give names to people ("Sanjaya", "Harini"), places ("Delhi", "Madurai"), things ("book", "pencil"), and ideas ("love", "freedom");
adjectives, which describe some noun ("happy," "all," "three", "eaten"); and
pronouns, which substitute for some noun ("it," "they", "I", "you").
Generally, Sanskrit noun endings express three different ideas: gender, case, and number.
Gender
Gender appears in English too, but only in a very limited way. "He" and "she" mostly mean the same thing and work the same way, but "she" is used for people and animals that seem female, and "he" is used for people and animals that seem male.
In Sanskrit, gender is used much more extensively. Almost all nominal words have gender, even if the gender doesn't make sense biologically. For example, words for trees are usually male (masculine), words for rivers are usually female (feminine), and so on.
Sanskrit also has a third neuter gender that is often used for objects and abstract ideas.
Case
Case is the name for how languages like Sanskrit show relationships between different words.
Sanskrit has eight cases. The first usually shows who or what is doing some action (the "subject"):
gajaḥ khādati. (The elephant eats.)
The second usually shows what is affected by some action (the "object"):
gajaḥ phalam khādati. (The elephant eats a fruit.)
The third means something like "with," "by," or "through":
gajo hastena phalaṃ khādati. (The elephant eats a fruit with its trunk.)
The fourth means something like "for" or "to":
gajo vānarāya phalaṃ dadāti. (The elephant gives a fruit to a monkey.)
The fifth means something like "from," "than," or "because of":
vānaro vṛkṣād gacchati. (The monkey goes away from its tree.)
The sixth means something like "of":
vānaro gajasya phalaṃ khādati. (The monkey eats the elephant's fruit.)
The seventh means something like "in," "on," or "at":
phalaṃ vānare. (The fruit is in the monkey.)
And the eight shows who or what the speaker wants to address:
he gaja! aham api phalam icchāmi! (Hey elephant! I want a fruit too!)
Number
Number appears extensively in English. The "number" of a word is just the number of things the word describes.
English uses two numbers: the singular ("dog", "cat"), which describes one thing, and the plural ("dogs", "cats"), which describes more than one thing.
In addition to the singular and the plural, Sanskrit also has a dual, which is used for exactly two things. In Sanskrit, the plural is used for three or more things.