Often times when using the Drupal Form API creating custom forms, you'll need to get a select list of Taxonomy terms as options. These snippets will help you look up terms in a vocabulary and put them in your options array.

Drupal 8

// Create empty options array
$options = array();

// Taxonomy vocabulary machine name
$taxonomy = 'tax_machine_name';

// Get all the taxonomy terms from the vocabulary
$tax_items = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadTree($taxonomy);

// Inject into the options array
foreach($tax_items as $tax_item) {
  $options[$tax_item->tid] = $tax_item->name;
}

// Create a select form element
$form['taxonomy_options'] = array(
    '#type' => 'select',
    '#options' => $options,
    '#required' => TRUE,
);

Drupal 7

// Create empty options array
$options = array();

// Taxonomy vocabulary machine name
$taxonomy = 'tax_machine_name';

// Get all the taxonomy terms from the vocabulary
$vocab = taxonomy_vocabulary_machine_name_load('tax_machine_name')->vid;
$tax_items = taxonomy_get_tree($vocab);

// Inject into the options array
foreach($tax_items as $tax_item) {
  $options[$tax_item->tid] = $tax_item->name;
}

// Create a select form element
$form['taxonomy_options'] = array(
    '#type' => 'select',
    '#options' => $options,
    '#required' => TRUE,
);

In Drupal 7, you might want to output a simple class when using the l() function to create a link

l('Link Text', 'path', array('attributes' => array('class' => 'custom-class')));