1) DNA is wrapped around histone proteins to form chromatin
2) The genes are the functional units, the segments between them are used for regulation
3) The genetic code is the language used, gene expression is how the cell uses the information, the activity of the genes.
Explanation:
1) DNA forms a DNA/protein complex called chromatin. It does this by wrapping around histone proteins. These histone proteins are usually present in the form of a nucleosome, which is a unit containing 2 copies of 4 histones (H2A, H2B, H3 and H4). The chromatin fibre at its most compact forms tightly coiled structures called chromosomes. These structures are only present during cell division. When the cell is in interphase (i.e. not dividing), the chromatin is not as tightly condensed, and instead the chromatin is more relaxed to allow the genes within to be expressed.
2) Genes are the functional units that the cell uses to make RNA and protein. The genes are first transcribed into RNA, which is processed and then translated into a polypeptide chain, which forms a complete protein that performs activities in the cell/tissue/organism. However, the whole genome does not form genes, there are regions that do not correspond to a gene. These regions are called "non-coding DNA" or sometimes even "junk DNA". However, that does not mean that these regions do not have important roles. The role of this DNA is usually in regulating the activity of the nearby genes. This DNA might contain important regulatory sequences such as promoters/enhancers/silencers that control how the gene is used by the cell, by for example, recruiting transcription factors or silencing proteins.
3) The genetic code is the language used by the cell. It explains how the cell can transcribe the information in the DNA, to RNA, process the RNA, and then translate the RNA into a polypeptide, and eventually a mature protein. In contrast, gene expression represents how the cell actually uses this information. Not all the genes are transcribed at the same time, instead, the activity of genes is carefully controlled to produce appropriate gene expression patterns, allowing the cell to properly perform its functions. Gene expression is hugely different between cells in an organism, for example the gene expression patterns of a muscle cell will be hugely different to that of a blood cell.
