Thanks for asking. I hope I'll be able to find the words to explain what a chromosome is, and what it does.
First, i'll answer the specific things you ask. "Thread-like" just means like a piece of cotton thread - very long and thin. "eukaryotic" is a scientific term for the type of cells found in many living things. Eukaryotic cells are found in all the living things you see around you - plants, birds, insects, mushroom - only some tiny invisible things called bacteria have a different sort of cell, but it's not really important just now.
Before we continue, here are some pictures, first of a pair of human chromosomes - it may not look very long and thin, but that's because it's all wrapped around itself so it will fit into the space it has - and then a picture of all the pairs of human chromosomes. Now you can see what i'm talking about..
DNA is the chemical in living things that passes on information about how the organism is made...how to make new cells, how to build hearts, and eyes, and skin...It's because you get some DNA from your mum and some from your dad that in some ways, you'll look a little like both of them. You may even look a little like your grandad or grandma, because around a quarter of your DNA is the same as theirs. I should point out that you don't actually have the same individual pieces of DNA as your parents, but DNA works a bit like an alphabet, and put all together, it will read the same kind of 'story' as theirs, with similar words, sentences and paragraphs.
Chromosomes are important because they are the individual pieces of DNA that you have. Humans have 23 different chromosomes, varying in length (you can see the different sizes in the photograph above), and we have two very similar copies of each of these chromosomes.
So chromosomes are long thin pieces of DNA that carry the instructions needed to build the bits of your body in the first place, and to keep it working throughout your life. Every cell in your body contains the same chromosomes, telling it how to do its part in making your body work. The proteins of the chromosome are basically there to keep the DNA safe, and to fold it up to make it fit properly.
Hope i've helped.
Thanks for asking, and hope you ask about anything else you don't quite
understand!
James
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James Cotton
Div. of Environmental and Evolutionary Biology, Institute of Biomedical and
Life Sciences
University of Glasgow
http://taxonomy.zoology.g
la.ac.uk/~jcotton
j.cotton@udcf.gla.ac.uk